Section 34. The next thing to consider is the distribution of the food material absorbed through the walls of the alimentary canal to the living and active parts of the body. This is one of the functions of the series of structures-- heart and blood-vessels, called the circulation, circulatory system, orvascular system. It is not the only function. The blood also carries the oxygen from the lungs to the various parts where work is done and kataboly occurs, and it carries away the katastases to the points where they are excreted-- the carbon dioxide and some water to the lungs, water and urea to the kidneys, sulphur compounds of some kind to the liver.
Section 35. Theblood(Figure 4,Sheet 2) is not homogeneous; under the low power of the microscope it may be seen to consist of--
(1.) a clear fluid, theplasma, in which float--(2.) a few transparent colourless bodies ofindefinite and changing shape, and having a central brighter portion, thenucleuswith a still brighter dot therein thenucleolus-- thewhite corpuscles(w.c.), and(3.) flat round discs,without a nucleus, thered corpuscles(r.c.), greatly more numerous than the white.
Section 36. The chyle of the lacteals passes, as we have said, by the thoracic duct directly into the circulation. It enters the leftvena cava superior(l.v.c.s.) near where this joins thejugular vein(ex.j.) (see Figure 1,Sheet 2, th.d.) and goes on at once with the rest of the blood to the heart. The small veins of the villi, however, which also help suck up the soluble nutritive material, are not directly continuous with the other body veins, the systemic veins; they belong to a special system, and, running together into larger and larger branches, form the lieno gastric (l.g.v.) and mesenteric (m.v.) veins, which unite to form theportal vein(p.v.) which enters the liver (l.v.) and there breaks up again into smaller and smaller branches. The very finest ramifications of this spreading network are called the (liver)capillaries, and these again unite to form at last thehepatic vein(h.v.) which enters thevena cava inferior(v.c.i.), a median vessel, running directly to the heart. This capillary network in the liver is probably connected with changes requisite before the recently absorbed materials can enter the general blood current.
Section 37. The student has probably already heard the terms vein and artery employed. In the rabbit a vein is a vessel bringing blood towards the heart, while an artery is a vessel conducting it away. Veins are thin-walled, and therefore flabby, a conspicuous purple when full of blood, and when empty through bleeding and collapsed sometimes difficult to make out in dissection. They are formed by the union of lesser factors. The portal breaks up into lesser branches within the liver. Arteries have thick muscular and elastic walls, thick enough to prevent the blood showing through, and are therefore pale pink or white and keep their round shape.
Section 38. Theheartof the rabbit is divided by partitions into four chambers: two upper thin-walled ones, theauricles(au.), and two lower ones, both, and especially the left, with very muscular walls, theventricles(vn.). The right ventricle (r.vn.) and auricle (r.au.) communicate, and the left ventricle (l.vn.) and auricle (l.au.).
Section 39. The blood coming from all parts of the body, partly robbed of its oxygen and containing much carbon dioxide and other katastases, enters the right auricle of the heart through three great veins, the median vena cava inferior from the posterior parts of the body, and the paired venae cavae superiores from the anterior. With the beating of the heart, described below, it is forced into theright ventricleand from there through thepulmonary artery(p.a.) seen in the figure passing under the loop of the aorta (ao.) to the lungs.
Section 40. Thelungs(lg. Figure 1,Sheet 1) are moulded to the shape of the thoracic cavity and heart; they communicate with the pharynx by thetrachea(tr. in Figure 1,Sheet 1) or windpipe, and are made up of a tissue of continually branching and diminishing air-tubes, which end at last in small air-sacs, thealveoli. The final branches of the pulmonary arteries, the lung capillaries, lie in the walls of these air-sacs, and are separated from the air by an extremely thin membrane through which the oxygen diffuses into, and the carbon dioxide escapes from, the blood.
Section 41. The mechanism ofrespirationwill be understood by reference to Figure 3,Sheet 2. It will be noted, in dissecting that the lungs have shrunk away from the walls of the thorax; this collapse occurs directly an aperture is made in the thorax wall, and is in part due to their extremeelasticity. In life the cavity of the thorax forms an air-tight box, between which and the lungs is a slight space, thepleural cavity(pl.c.) lined by a moist membrane, which is also reflected, over the lungs. The thorax wall is muscular and bony, and resists the atmospheric pressure on its outer side, so that the lungs before this is cut through are kept distended to the size of the thoracic cavity by the pressure of the air within them. Ininspiration(or breathing-in) the ribs are raised by the external intercostal (Anglice,between-ribs, e.i.c.m.) and other allied muscles, and the diaphragm (dia.) contracts and becomesflatter; the air is consequently sucked, in as the lungs follow the movement of the thorax wall. Inexpirationthe intercostals and diaphragm relax and allow the elastic recoil of the lungs to come into play. The thoracic wall is simultaneously depressed by the muscles of the abdominal area, the diaphragm thrust forwards, as the result of the displacement and compression of the alimentary viscera thus brought about. (r.r.r. in theFiguremark ribs.)
Section 42. The oxygen and carbon dioxide are not carried in exactly the same way by the blood. The student will know from his chemical reading that neither of these gases is very soluble, but carbon dioxide is sufficiently so in an alkaline fluid to be conveyed by the liquid plasma. The oxygen however, needs a special portative mechanism in the colouring matter of the red corpuscles, thehaemoglobin, with which it combines weakly to formoxy-haemoglobinof a bright red colour, and decomposing easily in the capillaries (the finest vessels between the arteries and veins), to release the oxygen again. The same compound occurs in all true vertebrata, and in the blood-fluid of the worm; in the crayfish a similar substance,haemocyanin, which when oxygenated is blue, and when deoxydized colourless, discharges the same function.
Section 43. The blood returns from the lungs to theleft auricle(l.au.) by the pulmonary veins, hidden in theFigureby the heart, passes thence to the thick-walledleft ventricle(l.vn.), and on into theaorta(ao.).
Section 44. The beating of the heart is, of course, a succession of contractions and expansions of its muscular wall. The contraction, orsystole, commences at the base of the venae cavae and passes to the auricles, driving the blood before it into the ventricles, which then contract sharply and drive it on into the aorta or pulmonary artery; a pause and then a dilatation, thediastolefollows. The flow of the blood is determined in one direction by the various valves of the heart. No valves occur in the opening of the superior cavae but an imperfect one, the Eustachian valve, protects the inferior cava; the direction of the heart's contraction prevents any excessive back-flow into the veins, and the onward, tendency is encouraged by the suck of the diastole of the ventricles. Between the left ventricle and auricle is a valve made up of two flaps of skin, themitral valve, the edges of the flaps being connected with the walls of the ventricle through the intermediation of small muscular threads,the chordae tendinae, which stretch across its cavity to little muscular pillars, thepapillary muscles; these attachments prevent the mitral valve from flapping back into the auricle, and as the blood flows into and accumulates in the ventricle it gets behind the flaps of the valve and presses its edges together. When the systole of the ventricle occurs, the increased, tension of the blood only closes the aperture the tighter, and the current passes on into the aorta, where we find three watch-pocket valves, with the pocket turned away from the heart, which are also closed and tightened by any attempt at regurgitation (back-flow). A similar process occurs on the right side of the heart, but here, instead of a mitral valve of two flaps between auricle and ventricle, we have atricuspid valvewith three. The thickness of the muscular walls, in view of the lesser distance through which it has to force the blood, -are- [is] less for the right ventricle than the left.
Section 45. The following are thechief branches of the aorta. The student should be able to follow them with certainty in dissection; they are all displayed in theFigure; but it must not be imagined for a moment that familiarity with this diagram will obviate the necessity for the practical work; (in.) is the innominate artery; it forks into (s.cl.a.) the right subclavian, and (r.c.c.) the right common carotid. Each carotid splits at the angle of the jaw into an internal and an external branch. The left common carotid, (l.c.c.) arises from the base of the innominate,* (l.s.cl.a.) the left subclavian, directly from the aorta. The aorta now curves round to the dorsal middle line, and runs down as seen in Figure 1,Sheet 1(d.ao.) and Figure 1,Sheet 2(d.ao.). Small branches are given off to the ribs, and then comes the mediancoeliac(coe.a.) to the stomach and spleen, the median superior mesenteric (s.mes.a.) to the main portion of the intestine, and the inferior mesenteric (p.m.a.) to the rectum. Note that no veins to the inferior vena cava correspond to these arteries-- the blood they supply going back by the portal vein (p.v.). The paired renal arteries (r.a.) supply the kidneys, and the common iliacs (c.il.a.) the hind legs, splitting into the internal iliacs (i.il.a.) and the femoral (f.).
{Lines from Second Edition only.}[The student should note that the only arteries in the middle line are those supplying the alimentary canal.]{Lines from First Edition only.}* -The figure is inaccurate, and represents the left common carotid as arising from the aortic arch.-
Section 46. The distribution of theveinsof the rabbit has only a superficial parallelism with arteries. The chief factors of vena cava inferior are the hepatic vein (h.v.), which receives the liver blood, the renal veins (r.v.), from the kidneys, the ilaeo-lumbar, from the abdominal wall, and the external (e.il.v.) and internal ilias (i.il.v.); with the exception of the renal veins none of these run side by side with arteries. The superior cavae (r. and l.v.c.s.) are formed by the union of internal (i.j.) and external jugular (e.j.) veins with a subclavian (s.cl.v.) from the fore limb. The termpre-caval veinis sometimes used for superior cava. The attention, of the student is called to the smallazygos vein(az.) running into the right vena cava superior, and forming the only asymmetrical (not-balancing) feature of the veins in front of the heart; it brings blood back from the ribs of the thorax wall, and is of interest mainly because it answers to an enormous main vessel, the right post-cardinal sinus, in fishes. There are spermatic arteries and veins (s.v. and a.) to the genital organs. All these vessels should be patiently dissected out by the student, and drawn.
Section 47. Between the final branches of the arteries and the first fine factors of the veins, and joining them, come thesystemic capillaries. These smallest and ultimate ramifications of the circulation penetrate every living part of the animal, so that if we could isolate the vascular system we should have the complete form of the rabbit in a closely-meshed network. It is in the capillaries that the exchange of gases occurs and that nutritive material passes out to the tissues and katastases in from them; they are the essential factor in the circulatory system of the mammal-- veins, arteries, and heart simply exist to remove and replace their contents. The details of the branching of the pulmonary artery and the pulmonary veins need not detain us now.
Section 48. Summarising the course of the circulation, starting from the right ventricle, we have-- pulmonary artery, pulmonary capillaries, pulmonary vein, left auricle, left ventricle, aorta, arteries, and systemic capillaries. After this, from all parts except the spleen and alimentary canal, the blood returns to systemic veins, superior or inferior cavae, right auricle, and right ventricle. The blood from the stomach spleen, and intestines however, passesvia{through} the portal vein to the liver capillaries and then through the hepatic vein to inferior cava, and so on. Material leaves the blood to be excreted in lungs, kidneys, by the skin (as perspiration), and elsewhere. New material enters most conspicuously;
(a) by the portal veins portal veins and(b) by the thoracic duct and left superior cava.
Section 49. The following table summarises what we have learnt up to the present of the physiology of the Rabbit, considered as a mechanism using up food and oxygen and disengaging energy:--
-Air_ {Nitrogen... returned unchanged.}{Oxygen... through Pulmonary Vein to--} {see 3.}-Food_ {Carbo-Hydrates (Starch, Sugar, Cellulose.)} Sugar.{Protein.} {Peptones.}{Fat (little in Rabbit.)} {Glycerine, and fatty acids in soups.}{Rejected matter got rid of in Defaecation.}1a. {Chyle in Lacteals goingvia{through} Thoracic Duct and LeftSuperior Cava to--} {see 2.}1b. {Veins of Villi--}{Portal Vein--}{Liver--}{Hepatic Vein and Inferior Cava to--} {see 2.}2. {Right side of heart; then to lungs, and then to--} {see 3.}3. {Left side of heart; whence to Systemic Arteries and Capillaries.}4. {The tissues and -Kataboly_.}5. {Urea (?Liver) Kidney and Sweat Glands}{CO2} {Lungs}{H2O} {Lungs, Kidney, Sweat Glands}{Other Substances} {Mainly by [Kidney,] Liver and Alimentary Canal}
Section 50. We have thus seen how the nutritive material is taken into the animal's system and distributed over its body, and incidentally, we have noted how the resultant products of the creature's activity are removed. The essence of the whole process, as we have already stated, is the decomposition and partial oxydation of certain complex chemical compounds to water, carbon dioxide, a low nitrogenous body, which finally takes the form of urea, and other substances. We may now go on to a more detailed study, the microscopic study, orhistology, of the tissues in which metaboly and kataboly occur, but before we do this it will be convenient to glance for a moment at another of our animal types-- theAmoeba, the lowest as the rabbit is the highest, in our series.
Section 51. This is shown in Figure III.,Sheet 3, as it would appear under the low power of the microscope. We have a mass of a clear, transparent, greyish substance called protoplasm, granular in places, and with a clearer border; within this is a denser portion called thenucleus, orendoplast(n.), which, under the microscope, by transmitted light, appear brighter, and within that a still denser spot, thenucleolus(ns.) orendoplastule. The protoplasm is more or less extensively excavated by fluid spaces,vacuoles; one clearer circular space or vacuole, which is invariably present, appears at intervals, enlarges gradually, and then vanishes abruptly, to reappear after a brief interval; this is called thecontractile vacuole(c.v.). The amoeba is constantly changing its shape, whence its older name of the Proteus animalcule, thrusting out masses of its substance in one direction, and withdrawing from another, and hence slowly creeping about. These thrust-out parts, in its outline, are calledpseudopodia(ps.). By means of them it gradually creeps round and encloses its food. Little particles of nutritive matter are usually to be detected in the homogeneous protoplasm of its body; commonly these are surrounded by a drop of water taken in with them, and the drop of water is then called afood vacuole. The process of taking in food is calledingestion. The amoeba, in all probability, performs essentially the same chemical process as we have summarised in Sections10,11,12; itingestsfood, digests it in the food vacuoles and builds it up into its body protoplasm, to undergo kataboly and furnish the force of its motion-- the contractile vacuole, is probably respiratory and perhaps excretory, accumulating and then, by its "systole" (compareSection 44), forcing out of its body, the water, carbon dioxide, urea, and other katastases, which are formed concomitantly with its activity. The amoeba reproduces itself in the simplest way; the nucleus occasionally divides into two portions and a widening fissure in the protoplasm of the animal's body separates one from the other. It is impossible to say that one is the parent cell, and the other the offspring; the amoeba we merely perceive,wasone and is now two. It is curious to note, therefore, that the amoeba is, in a sense, immortal-- that the living nucleus of one of these minute creatures that we examine to-day under a microscope may have conceivably drawn, out an unbroken thread of life since the remotest epochs of the world's history. Although no sexual intercourse can be observed, there is reason to believe that a process of supposed "cannabalism," in which a larger amoeba may occasionally engulph a smaller one, is really a conjugative reproductive process, and followed by increased vitality and division.
Section 52. Now if the student will compareSection 35, he will see that in the white blood corpuscles we have a very remarkable resemblance to the amoeba; the contractile vacuole is absent, but we have the protoplasmic body, the nucleus and nucleolus, and those creeping fluctuations of shape through the thrusting out and withdrawal of pseudopodia, which constitute "amoeboid" motion. They also multiply, in the same way, by division.
Section 53. It is not only in the white corpuscle of the blood that we find this resemblance; in all the firmer parts of the body we find, on microscopic examination, similar little blebs of protoplasm, and at an early stage of development the young rabbit is simplyone mass of these protoplasmic bodies. Their division and multiplication is an essential condition, of growth. Through an unfortunate accident, these protoplasmic blebs, which constitute the living basis of the animal body, have come to be styled "cells," though the term "corpuscles" is far more appropriate.
Section 54. The word is "cell" suggests something enclosed by firm and definite walls, and it was first employed in vegetable histology. Unlike the typical cells of animals, the cells of most plants are not naked protoplasm, but protoplasm enclosed in a wall of substance (cell wall) calledcellulose. The presence of this cellulose cell wall, and the consequent necessity of feeding entirely upon liquids and gases that soak through it instead of being able toingesta portion of solid food is indeed, theprimary distinction between the vegetable and the animal kingdoms, as ordinarily considered.
Section 55. Throughout life, millions of these cells retain their primary characters, and constitute the white corpuscles of blood, "phagocytes," and connective tissue corpuscles; others again, engage in the formation of material round themselves, and lie, in such cases, as gristle and bone, embedded in the substance they have formed; others again, undergo great changes in form and internal structure, and become permanently modified into, for instance, nerve fibres and muscle substance. The various substances arising in this way through the activity of cells are calledtissues, the building materials of that complex thing, the animal body. Since such a creature as the rabbit is formed through the co-operation of a vast multitude of cells, it is calledmulticellular; the amoeba, on the other hand, isunicellular. The rabbit may be thus regarded as a vast community of amoeboid creatures and their products.
Section 56. Figure IV.,Sheet 3represents, diagrammatically,embryonic tissue, of which, to begin with, the whole animal consists. The cells are all living, capable of dividing and similar, but as development proceeds, theydifferentiate, some take on one kind of duty (function), and some another, like boys taking to different trades on leaving school, and wide differences in structure and interdependence become apparent.
Section 57. It is convenient to divide tissues into three classes, though the divisions are by no means clearly marked, nor have they any scientific value. The first of these comprises tissues composed wholly, or with the exception of an almost imperceptible cementing substance, of cells; the second division includes the skeletal tissues, the tissue of mesentery, and the connective and basement tissue of most of the organs, tissues which, generally speaking, consist of amatrixor embedding substance, formed by the cells and outside of them, as well as the cells themselves; and, thirdly, muscular and nervous tissue. We shall study the former two in this chapter, and defer the third division until later.
Section 58. The outer layer of the skin (theepidermis), the inmost lining of the alimentary canal, the lining of the body cavity, and the inner linings of blood-vessels, glands, and various ducts constitute our first division. The general name for such tissues isepithelium. When the cells are more or less flattened, they formsquamous epithelium(Figure VI.) such as we find lining the inside of a man's cheek (from which the cells sq.ep. were taken) or covering the mesentery of various types-- sq.end. are from the mesentery (Section 16) of a frog. A short cylindroidal form of cell makes upcolumnar epithelium, seen typically in the cells covering the villi of the duodenum (Figure V.). This epithelium of the villi has the outer border curiously striated, and this is usually spoken of as leading towards "ciliated" epithelium, to be described immediately. The epithelium of the epidermis is stratified-- that is to say, has many thicknesses of cells; the deeper layers are alive and dividing (stratum mucosum), while the more superficial are increasingly flattened and drier as the surface is approached (stratum corneum) and are continually being rubbed off and replaced from below.
Section 59. In the branching air-tubes of the lung, the central canal of the spinal cord, and in the ureters of the rabbit, and in most other types, in various organs, we findciliated epithelium(Figure VII.). This is columnar or cubical in form, and with the free edge curiously modified and beset with a number of hair-like processes, thecilia, by which, during the life of the cell, a waving motion is sustained in one direction. This motion assists in maintaining a current in the contents of ducts which are lined with this tissue. The motion is independent of the general life of the animal, so long as the constituent cell still lives, and so it is easy for the student to witness it himself with a microscope having a 1/4-inch or 1/6-inch objective. Very fine cilia may be seen by gently scraping the roof of a frog's mouth (the cells figured are from this source), or the gill of a recently killed mussel, and mounting at once in water, or, better, in a very weak solution of common salt.
Section 60. The lining ofglandsissecretory epithelium; the cells are usually cubical or polygonal (8, g.ep.), and they display in the most characteristic form what is calledmetabolism. Anaboly (seeSection 14) we have defined, as a chemical change in an upward direction-- less stable and more complex compounds are built up in the processes of vegetable and animal activity towards protoplasm; kataboly is a chemical running down;metabolyis a more general term, covering all vital chemical changes. The products of the action of a glandular epithelium are metabolic products, material derived from the blood is worked, up within the cell, not necessarily with conspicuous gain or loss of energy, and discharged into the gland space. The most striking case of this action is in the "goblet cells" that are found among the villi; these are simply glands of one cell,unicellularglands, and inFigure V.we see three stages in their action: at g.c.1 material (secretion) is seen forming in the cell, at g.c.2 it approaches the outer border, and at g.c.3 it has been discharged, leaving a hollowed cell. Usually however, the escape of secreted matter is not so conspicuous, and the gland-cells are collected as the lining of pits, simple, as in the gastric, pyloric, and Lieberkuhnian glands (Figure VIII., Sections23,29), or branching like a tree or a bunch of grapes (Figure r.g.), as in Brunner's glands (Section 29) the pancreas, and the salivary glands. The salivary glands, we may mention, are a pair internal to the posterior ventral angle of the jaw, thesub-maxillary; a pair anterior to these, thesub-lingual; a pair posterior to the jaw beneath the ear, theparotid, and a pair beneath the eye, theinfra orbital.
Section 61. Theliveris the most complicated gland in the body (Figure X.). The bile duct (b.d.) branches again and again, and ends at last in the final pits, the lobuli (lb.), which are lined with secretory epithelium, and tightly packed, and squeeze each other into polygonal forms. The blood supply from which the bile would appear to be mainly extracted, is brought by the portal vein, but this blood is altogether unfit for thenutritionof the liver tissue; for this latter purpose a branch of the coeliac artery, the hepatic serves. Hence in the tissue of the liver we have, branching and interweaving among the lobuli, the small branches of the bile duct (b.d.), which carries away the bile formed, the portal vein (p.v.), the hepatic artery (h.a.), and the hepatic vein (h.v.). (CompareSection 45.) Figure X.b shows a lobule; the portal vein and the artery ramify round the lobules-- areinter-lobular, that is (inter, between); the hepatic vein begins in the middle of the lobules (intra-lobular), and receives their blood. (Compare X.a.) Besides its function in the manufacture of the excretory, digestive, and auxiliary bile, the liver performs other duties. It appears to act as an inspector of the assimilation material brought in by the portal vein. The villi, for instance, will absorb arsenic, but this is arrested and thrown down in the liver. A third function is the formation of what would seem to be a store of carbo-hydrate,glycogen, mainly it would appear, from the sugar in the portal vein, though also, very probably, from nitrogenous material, though this may occur only under exceptional conditions. Finally, the nitrogenous katastases, formed in the working of muscle and nerve, and returned by them to the blood for excretion, are not at that stage in the form of urea. Whatever form they assume, they undergo a further metabolism into urea before leaving the body, and the presence of considerable quantities of this latter substance in the liver suggests this as a fourth function of this organ-- the elaboration of urea.
Section 62. Similar from a physiological point of view, to the secretory glands which form the digestive fluids are those which furnish lubricating fluids, the lachrymal gland, and Harderian glands in the orbit internally to the eye, and posterior and anterior to it respectively, the sebaceous glands (oil glands) connected with the hair, and the anal and perineal glands. The secretions of excretory glands are removed from the body; chief among them are the sweat glands and kidneys. The sweat glands are microscopic tubular glands, terminating internally in a small coil (Figure VIII.s.g.) and are scattered thickly over the body, the water of their secretion being constantly removed by evaporation, and the small percentage of salt and urea remaining to accumulate as dirt, and the chief reasonable excuse for washing. The kidney structure is shown diagrammatically in Figure 5,Sheet 7. A great number of branching and straight looped,tubuli(little tubes) converge on an open space, thepelvis. Towards the outer layers (cortex) of the kidney, these tubuli terminate in little dilatations into which tangled knots of blood-vessels project: the dilatations are called Bowman's capsules (B.c.), and each coil of bloodvessel a glomerulus (gl.). In the capsules, water is drained from the blood; in the tubuli, urea and other salts in the urine aresecretedfrom a branching network of vessels.
Section 63. In all the epithelial tissues that we have considered we have one feature in common: they are cells, each equivalent to the amoeba, that have taken on special duties-- in a word, they are specialists. The amoeba is Jack of all trades and a free lance; the protective epidermal cell, the current-making ciliated cell, the bile or urea-making secretory cell, is master of one trade, and a soldier in a vast and wonderfully organized host. We will now consider our second kind of cell in this organization, the cell of which the especial aim is the building round it of a tissue.
Section 64. The simplest variety in this group ishyaline(i.e. glassy)cartilage(gristle). In this the formative cells (the cartilage corpuscles) are enjellied in a clear structureless matrix (Figure XII.), consisting entirely of organic compounds accumulated by their activity. Immediately round the cell lies acapsuleof newer material. Some of the cells have recently divided (1); others have done so less recently, and there has been time for the interpolation of matrix, as at 2. In this way the tissue grows and is repaired. A thin layer of connective tissue (see below), theperichondrium, clothes the cartilaginous structure.
Section 65.Connective tissue(Figure XIII) is a general name for a group of tissues of very variable character. It is usually described as consisting typically in the mammals of three chief elements felted together; of comparatively unmodified corpuscles (c.c.), more or less amoeboid, and of fibres which are elongated, altered, and distorted cells. The fibres are of two kinds: yellow, branching, and highlyelastic(y.e.f.), in consequence of which they fall into sinuous lines in a preparation, and white andinelasticones (w.i.f.), lying in parallel bundles. Where the latter element is entirely dominant, the connective tissue istendon, found especially at the point of attachment of muscles to the parts they work. Some elastic ligaments are almost purely yellow fibrous tissue. A loose interweaving of the three elements isareolar tissue, the chief fabric of mesentery, membrane, and thedermis(beneath the epidermis). With muscle it is the material of the walls of the alimentary canal and bloodvessels, and generally it enters into, binds together, and holds in place other tissue. The connective tissue of fishes displays the differentiation of fibres in a far less distinct manner.
Section 66. Through connective tissues wander thephagocytes, cells that are difficult to distinguish, if really distinct, from the white blood corpuscles. These cells possess a remarkable freedom; they show an initiative of their own, and seem endowed with a subordinate individuality. They occur in great numbers in a tissue called,botryoidal tissue(Figure XIV.), which occurs especially in masses and patches along the course of the alimentary canal, in its walls. The tonsils, swellings on either side of the throat, are such masses, and aggregates occur as visible patches, the Peyer's patches, on the ileum. It also constitutes the mass of the vermiform appendix and the wall of the sacculus rotundus; and in the young animal the "thymus gland," ventral to the heart, and less entirely, the "thyroid gland," ventral to the larynx, are similar structures, which are reduced or disappear as development proceeds. It is evident that in these two latter cases the term "gland" is somewhat of a misnomer. The matrix of botryoidal tissue is a network of stretched and hollowed connective tissue cells-- it is not a secretion, as cartilage matrix appears to be. During digestion, the phagocytes prowl into the intestine, and ingest and devourbacteria, that might otherwise give rise to disease. In inflammation, we may note here, they converge from all directions upon the point wounded or irritated. They appear to be the active agents in all processes of absorption (seeosteoclastsunderbone), and for instance, migrate into and devour the tissue of the tadpole's tail, during its metamorphosis to the adult frog.
Section 67. Within the connective tissue cellsfatdrops may be formed, as inFigure XV.Adipose tissueis simply connective tissue loaded with fat-distended cells. The tissue is, of course, a store form of hydro-carbon (Section 17) provided against the possible misadventure of starvation. With the exception of some hybernating animals, such store forms would seem to be of accidental importance only among animals, whereas among plants they are of invariable and necessary occurrence.
Section 68. We now come toBone, a tissue confined to the vertebrata, and typically shown only in the higher types. As we descend in the scale from birds and mammals to lizards, amphibia (frogs and toads) and fish, we find cartilage continually more important, and the bony constituent of the skeleton correspondingly less so. In such a type as the dog-fish, the skeleton is entirely cartilaginous, bone only occurs in connection with the animal's scales; it must have been in connection with scales that bone first appeared in the vertebrate sub-kingdom. In the frog we have a cartilaginous skeletonoverlaidby numerous bony scutes (shield-like plates) which, when the student comes to study that type, he will perceive are equivalent to the bony parts of such scales as occur in the dog-fish, sunk inward, and plating over the cartilage; and in the frog the cartilage also is itself, in a few places, replaced by bony tissue. In the adult rabbit these two kinds of bone, the boneoverlyingwhat was originally cartilage (membrane bone), and the bone replacing the cartilage (cartilage bone) have, between them, practically superseded the cartilage altogether. The structure of the most characteristic kind of bone will be understood by reference toFigure XVI. It is a simplified diagram of the transverse section of such a bone as the thigh bone. M.C. is the central marrow cavity, H.v., H.v. are cross sections of small bloodvessels, the Haversian vessels running more or less longitudinally through, the bone in canals, the Haversian canals. Arranged round these vessels are circles of the formative elements, the bone corpuscles orosteoblasts(b.c.) each embedded in bony matrix in a little bed, thelacuna, and communicating one with another by fine processes throughcanaliculiin the matrix, which processes are only to be seen clearly in decalcified bone (SeeSection 70). The osteoblasts are arranged in concentric series, and the matrix is therefore in concentric layers, orlamellae(c.l.). Without and within the zone of Haversian systems are (o.l. and i.l.), the outer and inner lamellae. The bone is surrounded by connective tissue, theperiosteum. In addition to thiscompact bone, there is a lighter and looser variety in which spicules and bars of bony tissue are loosely interwoven. Many flat bones, the bones of the skull, for instance, consist of thisspongy bone, plated (as an electro spoon is plated) with compact bone.
Section 69. Among the bony bars and spicules ofspongy boneoccurs the red marrow-- which must not be confused with the yellow marrow, the fatty substance in the central cavity of long bones. In this red marrow are numerous large colourless cells, which appear to form within their substance and then liberate red blood corpuscles. This occurs especially in the spongy bone within the ribs.
Section 70. The matrix of bone differs from that of cartilage or of most other tissues in consisting chiefly ofinorganicsalts. The chief of these is calcium phosphate, with which much smaller quantities of calcium carbonate, and magnesium phosphate and carbonate occur. These inorganic salts can be removed by immersion of the bone in weak hydrochloric acid, and a flexible network of connecting tissue, Haversian vessels, bone corpuscles, and their processes remains. This isdecalcified bonealluded to above.
Section 71. In the very young rabbit, the limb bones, vertebral column, and many of the skull bones are simply plates and bars of cartilage; the future membrane bones, however are planned out in connective tissue. Thedevelopmentof the latter is simple, the connective tissue corpuscles functioning by a simple change of product as osteoblast. The development of the cartilage bones, however, is more complicated.Figure XVII., represents, in a diagrammatic way, the stages in the conversion of a cartilaginous bar to bone. To begin with, the previously sporadically-arranged (scattered anyhow) corpuscles (u.c.c.) are gathered into groups in single file, or in other words, into "columnar" groups (as at c.c.). The matrix becomes clouded with inorganic salts of lime, and it is then said to becalcified. This calcified cartilage then undergoes absorption-- it must not be imagined for a moment that bone is calcified cartilage. Simultaneous with the formation of the cavities (s.) due to this absorption, connective tissue (p.c.i.) from the surrounding perichondrium (p.c.) grows into the ossifying* bar. It is from this connective tissue that the osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is continually being absorbed and reformed by the activity of the osteoblasts. An osteoblast engaged in the absorption instead of the formation of bone is called anosteoclast.
* The formation of bone is calledossification. To ossify is to become bony.
Section 72. The great thing to notice about this is that cartilage does not become bone, but is eaten into and ousted by it; the osteoblasts and osteoclasts replace entirely the cartilage corpuscles, and are not derived from them.
Section 73. We may mention here the structure of thespleen(Figure 1,Sheet 1). It consists of a connective tissue and muscular coating, with an internal soft matrix much resembling botryoidal tissue, traversed by fibroustrabeculae(= beams, planks) containing blood-vessels, and the whole organ is gorged with blood, particularly after meals. The consideration of its function the student may conveniently defer for the present.
Section 74. Here also, we may notice thelymphatics, a series of small vessels which return the overflow of the blood serum from the capillaries, in the nutrition of the tissues in all parts of the body, to the thoracic duct (seeSection 36), and the general circulation. At intervals their course is interrupted by gland-like dilatations, thelymphatic glands, in which masses of rapidly dividing and growing (proliferating) cells occur, of which, doubtless, many are detached and become, first "lymph corpuscles," and, when they reach the veins, white blood corpuscles.
Section 75. We are now in a position to study the rabbit's skeleton. We strongly recommend the student to do this with the actual bones at hand-- they may be cleared very easily in a well-boiled rabbit. This recommendation may appear superfluous to some readers, but, as a matter of fact, the marked proclivity of the average schoolmaster for mere book-work has put such a stamp on study, that, in nine cases out of ten, a student, unless he is expressly instructed to the contrary, will go to the tortuous, and possibly inexact, descriptions of a book for a knowledge of things that lie at his very finger-tips. We have not written, this chapter to give a complete knowledge of the skeleton, but simply as an aid in the actual examination of the bones.
Section 76. We may take the skeleton under five headings. There is the central axis, the chain of little bones, the vertebrae, threaded on the spinal cord (see Figure 1 andSection 1); the thorax, the box enclosed by ribs and sternum; the fore-limb and bones connected with it (pectoral girdle and limb), and the hind-limb and bones connected with it (pelvic girdle). Finally there is the skull, but following the London University syllabus, we shall substitute the skull of the dog for of that of the rabbit, as more typically mammalian (Section 4).
Section 77. InSection 3(which the student should refer to) we have a division of the vertebrae into four varieties. Of these most representative is the thoracic. Athoracic vertebra(Figure 4,Sheet 5, T.V.) consists of a central bony mass, thebodyorcentrum(b.), from which there arises dorsally an arch, the neural arch (n.a.), completed by a keystone, the neural spine (n.s.); and coming off laterally from the arch is the transverse process (tr.p.). Looking at the vertebra sideways, we see that the arch is notched, for the exit of nerves. Jointed to the thoracic vertebrae on either side are the ribs (r.). Each rib has a process, thetuberculum, going up to articulate with the transverse process, and one, thecapitulumarticulating between the bodies of two contiguous vertebrae. The facets for the articulation of the capitulum are indicated in the side view by shading. At either end of the body of a vertebra of a young rabbit are bony caps, theepiphyses(ep.), separated from the body by a plane of unossified cartilage (indicated, by the dots). These epiphyses to the vertebral bodies occur only among mammals, and are even absent in some cases within the class. In the adult rabbit they have ossified continuously with the rest of the body.
Section 78. Acervical vertebra(C.V.) seems, upon cursory inspection, to have no rib. The transverse processes differ from those of thoracic series in having a perforation, thevertebrarterial canal, through which the vertebral artery runs up the neck. A study of the development of these bones shows that the part marked f.r. ossifies separately from the rest of the transverse process; and the form of the equivalent structures in certain peculiar lower mammals and in reptiles leaves no doubt that f.r. is really an abbreviated rib; fused up with the transverse process and body. The two anterior cervical vertebrae are peculiar. The first (at.) is called theAtlas-- the figure shows the anterior view-- and has great articular faces for the condyles (Section 86) of the skull, and a deficient centrum. The next is the axis, and it is distinguished by anodontoid peg(od.p.), which fits into the space where the body of the atlas is deficient. In development the centrum of the axis ossifies from one centre, and the odontoid, peg from another, which at that time occupies the position of centrum of the atlas. So that it would seem that the atlas is a vertebraminusa centrum, and the axis is a vertebraplusa centrum, added at the expense of the atlas.
Section 79. Thelumbar vertebrae(l.v.) are larger, and have cleft transverse processes, each giving rise to an ascending limb, themetapophyses, and a descending one. The latter (generally spoken of asthetransverse processes) point steeply downward, and are considerably longer than those of thoracic series. Thesacral vertebrae(s.v.) have great flattened transverse processes for articulation with the ilia. Thecaudal vertebrae(c.v.) are gradually reduced to the mere elongated centra, as we proceed, towards the tip of the tail.
Section 80. All the vertebrae join with their adjacent fellows through the intermediation of certainintervertebral pads, and also articulate by small processes at either end at the upper side of the arch, thezygapophyses. The normals to the polished facets of these point, in the case of the anteriorzygapophyses, up and in (mnemonic:ant-up-in), and in the case of the posterior, down and out. The student should make this, and the other features of vertebrae, out upon actual specimens.
Section 81. Thethoraxis bounded dorsally by the vertebral column, and ventrally by the sternum. The sternum consists of segments, thesternebrae(st.); anteriorly there is a bonymanubrium(mb.), posteriorly a thin cartilaginous plate, thexiphisternum(xi.). Seven pairs of ribs articulate by cartilaginous ends (sternal ribs) with the sternum directly, as indicated in the figure; five (false) ribs are joined, to each other and to the seventh, and not to the sternum directly. The last four ribs have no tuberculum (Section 77).
Section 82. The fore-limb (pectoral limb) consists of an upper arm bone, thehumerus(hum.) the distal end of which is deeply excavated by the olecranon fossa (o.f.) as indicated by the dotted lines; of two bones, the ulna (u.) and radius (r.) which are firmly bound by ligament in the position of the figure (i.e., with the palm of the hand downward, "prone"); of a number of small bones (carpalia), thecarpus(c.); of a series ofmetacarpals(mc.); and of three digits (= fingers) each, except the first, orpollex, of three small bones-- thephalanges, only the proximal of which appear in the figure. The ulna has a hook-like head, theolecranon(o.) which distinguishes it easily from the distally thickened radius. The limb is attached to the body through the intermediation of the shoulder-blade (scapula, sc.) a flattened bone with a median external ridge with a hook-like termination, theacromion(acr.). There is also a process overhanging theglenoid cavity(g.) wherein the humerus articulates, which process is calledcoracoid(co.); it is ossified from two separate centres, and represents a very considerable bone in the bird, reptile, and frog. Along the dorsal edge of the scapula of the rabbit is unossified cartilage, which is called thesupra-scapula(s.sc.). In man there runs from the acromion to the manubrium of the sternum a bone, the collar-bone orclavicle. This is represented by a very imperfectly ossified rudiment in the rabbit. The scapula and clavicle, the bones of the body connected with the fore-limb, are frequently styled thepectoral girdle, orshoulder-girdle; this name of girdle will appear less of a misnomer when lower vertebrate types are studied.
Section 83. The hind limb and its body bones--pelvic limb and girdle-- are shown inFigure 2.The limb skeleton corresponds closely with that of the fore-limb. Thefemur(fe.) answers to thehumerus, and is to be distinguished from it by the greater distinctness of its proximal head (hd.) and by the absence of an olecranon fossa from its distal end. Thetibia(ti = theradius) is fused for the distal half of its length with thefibula(fb. =ulna). Atarsus(tarsalia) equals thecarpus.* Two of the proximal tarsalia may be noted: one working like a pulley under the tibia, is theastragalus(as.); one forming the bony support of the heel, is thecalcaneum(ca.). There is a series of metatarsals, and then come four digits of three phalanges each.
* Such a resemblance as exists between one vertebra and another in the rabbit, or between the humerus and the femur, is calledserial homology; the two things correspond with each other to the extent of imperfect reduplication. "Homology" simply is commonly used to indicate the resemblance between any two structures in different animals, in origin and position as regards other parts. Thus the heart of the rabbit and of the frog are homologous structures, corresponding in position, and resembling each other much as two memory sketches of one picture might do.
Section 84. The pelvic girdle differs from the pectoral in most land vertebrata in beingarticulated with the vertebral column. This difference does not exist in fishes. It consist in the rabbit of four bones; the ilium (i.), the ischium (is.), the pubis (pb.), and the small cotyloid bone-- the first two and the latter one meeting in the acetabular fossa (ac.) in which the head of the femur works. The pubes and ischia are fused along the mid-ventral line. Many morphologists regard, the ilium as equivalent to, that is, strictly corresponding in its relation, to the scapula, the pubis to the cartilaginous substratum of the clavicle, and the ischium to the coracoid.
Section 85. These bones will be studied at the greatest advantage when dissected out from a boiled rabbit. Prepared and wired skeletons, disarticulated skeletons, plates of figures, and written descriptions are in succession more tedious and less satisfactory ways to a real comprehension, of this matter. This chapter directs the student's attention to the most important points in the study of the skeleton, but it is in no way intended to mitigate the necessity of practical work. It is a guide simply.
Section 86. The mammalian skull will be better understood after the study of that of some lower vertebrate. We shall describe its main features now, but their meaning will be much clearer after the lower type is read. Our figures are ofCanis. In section (Figure VI.,Sheet 6), we perceive a brain case (cranium) opening behind by a large aperture, theforamen magnum(F.M.). In front of this is an extensive passage, thenasal passage(E.N. to P.N.) which is divided from the mouth by a bony floor, thepalate, and which opens into the pharynx behind at theposterior nares(P.N.) and to the exterior by the anterior orexternal nares(E.N.). It is divided into right and left passages by a middle partition, the nasal septum. Outside the skull, on its wings, is a flask-like bone, thebulla tympani(b. inFigures 2 and 3), protecting the middle ear, and from above this there passes an arch, the cheek bone (ju. inFigures 1, 2, and 3), to the upper jaw, forming in front the bony lower protection of the cavity containing the eye, theorbit. The cheek arch, nasal passage, and jaws, form collectively the "facial apparatus," as distinguished from the cranium, and the whole skull is sometimes referred to as, the "cranio-facial apparatus." Two eminences for articulation with the atlas vertebra, thecondyles(c.), lie one on each side of the lower boundary of the foramen magnum.
Section 87. The floor of thecraniumconsists of a series of cartilage bones, thebasi-occipital(b.o.),basi-sphenoid(b.sp.),pre-sphenoid(p.sp.), and in front, theethmoid(eth.), which sends down a median plate, not shown, in the figure, to form the nasal septum between right and left nasal passages. Like extended wings on either side of the basi-occipital are theex-occipital(e.o.) (the bone is marked inFigure 4, but the letters are a little obscured by shading). Similarly theali-sphenoids(a.s.), are wings to thebasi-, and theorbito-sphenoids(o.s.), to thepre-sphenoidbone (p.sp.). Between the ex-occipital and ali-sphenoid there is wedged in a bone, theperiotic(p.o.) containing the internal ear (Section 115). Above the foramen magnum the median supra-occipital bone completes what is called the occipital arch. Apairofparietals(pa.) come above the ali-sphenoids, and a pair offrontals(f.) above the orbito-sphenoids. At the side the brain case is still incomplete, and here the squamosal (sq.) enters into its wall. In the external view (Figure 3) the bulla hides the periotic bone from without. The student should examine all four figures for these bones before proceeding.
Section 88. The outer edge of theupper jawand the cheek arch are made up of three paired bones. First comes the premaxilla (p.m.) (not p.m.1 or p.m.4), containing in the dog, the three incisors of either side. Then comes the maxilla, bearing the rest of the teeth.* Thejugalor malar (ju.) reaches over from the maxilla to meet azygomatic process(= connecting outgrowth) (z.p.) of the squamosal bone.
* In the dog a sabre-like canine (c.), four premolars (p.m.1 and p.m.4) and two molars (m.1 and m.2).
Section 89. In the under view of the skull (Figure 2) it will be seen that the maxilla sends in a plate to form the front part of thehard palate. Behind, the hard palate is completed by the pair of palatine bones (pal.), which conceal much of the pre- and orbito-sphenoid in the ventral view, and which run back as ridges to terminate in two small angular bones, the pterygoids (pt.) which we shall find represent much more important structures in the lower vertebrata.
Section 90. The pre-maxillae and maxillae bound the sides of thenasal passage, and it is completed above by a pair of splints, the nasals. Along the floor of the nasal passage, on the middle line, lies a splint of bone formed by the coalescence of two halves. It embraces in aV-like groove the mesethmoid (nasal septum) above, and lies on the palate.
{Lines from First Edition only.}-Its position is indicated by a heavy black line in 4, and it iscalled, thevomerbone (vo.).-{Lines from Second Edition only.}[In the frog it is represented by two laterally situated bones. This isthevomerbone (vo.).]
The nasal passages are partially blocked by foliated bony outgrowths, from the inner aspect of their walls, which in life are covered with mucous membrane, and increase the surface sensitive to smell. The ethmoid ends in theethmo-turbinal(e.t.); the nasal, thenaso-turbinal(n.t.); and the maxilla, themaxillo-turbinal(m.t.). In the anterior corner of the orbit there is a bone, thelachrymal(lc.Figure 1), which is hidden by the maxilla in the side view of the skull.
Section 91. Thelower jaw(mandible) is one continuous bone in the mammal. Three incisors bite against the three of the upper jaw. Then comes a canine, four premolars, andthreemolars, the first of which is blade-like (sectorial tooth), and bites against the similar sectorial tooth (last premolar) of the upper jaw. The third molar is small. The arrangement of tooth is indicated in the following dental formula:-- I. 3.3/3.3, C. 1.1/1.1, P.M. 4.4/4.4, M. 2.2/3.3
Section 92. Attached just behind the bulla above, and passing round on either side of the throat to meet at the base of the tongue, is thehyoid apparatus(Figure 6). The stylohyal (s.h.), epihyal (e.h.), andceratohyal(c.h.) form the anterior cornu of the hyoid. Thebody of the hyoid(b.h.) forms a basis for the tongue. The posterior coruna (t.h.) of the hyoid are also called the thyrohyals.
Section 93. The following table presents these bones in something like their relative positions. A closer approximation to the state of the case will be reached if the student will imagine the maxilla raised up so as to overlie and hide the palatine and presphenoid, the squamosal similarly overlying the periotic bone, and the jugal reaching between them. Membrane bones are distinguished by capital letters.
-Cranium_-Nasal_ (paired),Ethmoid Bone(median), -Vomer_-Frontal_ (paired), -Lachrymal_ (paired),Orbito-sphenoid(paired),Pre-sphenoid(median), Eye-Parietal_ (Paired),Ali-sphenoid(paired),Basi-sphenoid(median)*,Periotic Bone(paired)-Bulla_ (paired)Supra-occipital(median),Ex-occipital(paired),Basi-occipital(median)-Upper Jaw_-Pre-Maxilla_ (paired)Palatine(paired)Pterygoid(paired)-Lower Jaw_-Maxilla_ (paired)-Jugal_ (paired)-Squamosal_ (paired)*In this table the small bones of the ear are simply indicated by an asterisk.
-Nasal_ (paired),Ethmoid Bone(median), -Vomer_-Frontal_ (paired), -Lachrymal_ (paired),Orbito-sphenoid(paired),Pre-sphenoid(median), Eye-Parietal_ (Paired),Ali-sphenoid(paired),Basi-sphenoid(median)*,Periotic Bone(paired)-Bulla_ (paired)Supra-occipital(median),Ex-occipital(paired),Basi-occipital(median)
-Pre-Maxilla_ (paired)Palatine(paired)Pterygoid(paired)
-Maxilla_ (paired)-Jugal_ (paired)-Squamosal_ (paired)
*In this table the small bones of the ear are simply indicated by an asterisk.
Section 94. Hidden by the bulla, and just external to the periotic bone, are theauditory ossicles, theincus,malleus,os orbiculare, andstapes. These will be more explicitly treated when we discuss the ear.
Section 95. When we come to the study of the nerves, we shall revert to the skull, and treat of its perforations. The student should not fail, before proceeding, to copy and recopy our figures, and to make himself quite familiar with them, and he should also obtain and handle an actual skull. For all practical purposes the skull of a sheep or cat will be almost as useful as that of the dog.