Fig. 408.—Aphrophora spumaria, Cuckoo-spit.a.The frothy substance;b.The pupa.
Fig. 408.—Aphrophora spumaria, Cuckoo-spit.
a.The frothy substance;b.The pupa.
TheAphrophora bifasciata, common frog-hopper, is a well-known garden pest. The antennæ of this insect are placed between the eyes, and the scutellum is not covered; the eyes, never more than two in number, are occasionally wanting. These pests are furnished with long hind legs, that enable them to perform most extraordinary leaping feats. The best-known British species is the cuckoo-spit, froth-fly (Fig. 408). The names cuckoo-spit and froth-fly both allude to the peculiar habit of the insect, while in the larva state, of enveloping itself in a kind of frothy secretion, somewhat resembling saliva.
Arachnidæ.—In this class of insects, spiders, scorpions, and mites are included, all of which belong to a sub-class of Arthropoda, and are appropriately placed between the Crustacea on the one hand and the Insecta on the other. The highest Crustaceans have ten feet, the Arachnidæ eight, and insects six. The Arachnidæ are wingless, have no antennæ, and breathe by means of tracheal tubes, or pulmonary sacs, these performing the function of lungs. As a rule they have several simple eyes, have no proper metamorphosis, and they are essentially predaceous, the females being larger than the males. Most of the Arachnidæ live on insects, and may therefore be regarded in the light of a friend to the florist and gardener.
TheEpeira diademais the best known member of the species; in summer spiders abound on every shrub, and spin out their wonderful webs from branch to branch.
Fig. 409.—A lengthways section through the body of femaleEpeira diadema.Explanation of reference.—ey.Eyes;p.g.Poison gland;ht.Heart;in.Intestine, alimentary canal;l.Liver;r.Rectum or cloaca;dt.andsp.Discharge tubes of spinnerets;o.Slit, or air opening;ov.Ovipositor;ph.Pharynx;br.Brain;thr.Throat, or gullet, filled with eggs;un. l.Under lip;m.Mouth;f.Fang, or claw;j.Jaw. The gills, or breathing apparatus are situated at the air opening,o; and the silk glands are above this. (Magnified 20 diameters.)
Fig. 409.—A lengthways section through the body of femaleEpeira diadema.
Explanation of reference.—ey.Eyes;p.g.Poison gland;ht.Heart;in.Intestine, alimentary canal;l.Liver;r.Rectum or cloaca;dt.andsp.Discharge tubes of spinnerets;o.Slit, or air opening;ov.Ovipositor;ph.Pharynx;br.Brain;thr.Throat, or gullet, filled with eggs;un. l.Under lip;m.Mouth;f.Fang, or claw;j.Jaw. The gills, or breathing apparatus are situated at the air opening,o; and the silk glands are above this. (Magnified 20 diameters.)
The body, seen in my illustration,Fig. 409, in section, consists of two parts; the foremost is the cephalothorax, or head, upon which is mounted four pairs of eyes (two of which are seen in section), while to the thorax is attached eight jointed well-developed legs terminating in feet, with claws adapted for climbing and holding on. The other half consists, of the abdomen, together with spinnerets and glands, which secrete the fluid out of which the web is spun, and this, although it hardens to some extent on exposure to the air, retains its viscid nature for the purpose of entangling its prey. The spinnerets are the most interesting feature in the anatomy of the Epeira (Figs. 410 and 411).
Fig. 410.1. Spinnerets of Spider; 2. Extreme end of one of the upper pair of spinnerets; 3. End of under pair of spinnerets; 4. Foot of Spider; 5. Side view of eye; 6. The arrangement of the four pairs of eyes.
Fig. 410.
1. Spinnerets of Spider; 2. Extreme end of one of the upper pair of spinnerets; 3. End of under pair of spinnerets; 4. Foot of Spider; 5. Side view of eye; 6. The arrangement of the four pairs of eyes.
Five kinds of spinning glands are found in spiders. The glandulæ aciniformes are those which consist of a proper tunica and an epithelium; these exhibit in all parts the same reaction to staining agents. The glandulæ pyriformes consist of a tunica proper and an epithelium,which in their lower parts (or those near the efferent ducts) stain more deeply than the upper. The glandulæ ampullaceæ and glandulæ tubuliformes have similar coverings, the latter terminating in a large spool. The glandulæ aggregatæ have a wide and branched lumen, the efferent duct of which is provided with cells and an accessory piece, which draws out to a tip. All the glands have secreting portions, which serve as collecting cavities for the spinning material. The spools are two-jointed basal and one-jointed accessory pieces. In addition to the five glands enumerated, there are also lobate and cribelleum glands; these are variously distributed, and exercise different functions, one set preparing the so-called moist filaments from the moist droplets, another spins the egg-cocoon, as nearly all spiders envelop their eggs in a covering of silken threads and store them up in some sheltered place awaiting the warm weather of spring to hatch them out. The bag that holds the eggs is not one of the least curious efforts of skill and care. The mother uses her body as a gauge to measure her work, precisely as a bird uses her body to gauge the size and form of its nest. The spider first spreads a thin coating of silk as a foundation, taking care to have this circular by turning its body round during the process. In the same manner it spins a raised border round this till it takes the form of acup; it is at this stage of the work the female begins to lay her eggs in the cup, and not content to fill it up to the brim, she also piles up a heap as high as the cup is deep. Here, then, is a cup full of eggs, the under half covered and protected by the silken sides of the cup, but the upper still exposed to the air and the cold. She now sets to work to cover this; the process is similar to the preceding—that is, she weaves a thick web of silk all round the top, and instead of a cup-shaped nest, like those of the bird tribe, the whole partakes of the form of a ball much larger than the body of the spider.
Fig. 411.—Spinnerets of Spider greatly enlarged.
Fig. 411.—Spinnerets of Spider greatly enlarged.
The eight legs and feet of the spider (one only is representedFig. 410, No. 4) are curiously constructed. Each foot, when magnified, is seen to be armed with strong horny claws, with serrations on their under-surface. By this arrangement the spider is enabled to regulate the issue of its web from the spinnerets. In addition, a remarkable comb-like claw is provided for the purpose of separating certain threads which enter into the composition of the delicate web, so that everything is arranged and planned in the most geometrical order, while the mouth or jaws with their two movable poison-fangs convert the Arachnidæ into formidable and dangerous foes. The maternal industry and instincts of spiders, the ballooning habits of others, the cave dwellers, with their limited vision, combined with an increased delicacy of touch and hearing, their disguise of feigned death when a strong enemy approaches, are all of the most interesting character.
One of the more remarkable, theArgyroneta aquatica(diving spider), weaves itself a curious little bell-shaped globule, which it takes with it to the bottom of the water, whither it retires to devour its prey. Notwithstanding its aquatic habits, this, like the rest of its species, is fitted only for aerial respiration; it therefore carries down, entangled amongst the hairs of its body,a small bubble of air. This contrivance presents us with the earliest form of diving-bell.
Mites and Ticksconstitute a group which for diversity of structure, number of species and individuals, and minuteness of size, has no equal. The typical genus of the family—Ixodidæ—being wholly parasitic in their habits, are so modified in organisation, so marked by degeneration, that some authors have proposed to remove them into a class by themselves. One leading character distinguishes the whole: the abdomen rarely presents a trace of segmentation, but is confluent with the cephalothorax, the fusion between the two being so complete that, as in the harvest spiders belonging to Palpatores, the anterior sternal plates of the abdomen are thrust far forward between the coxæ of the cephalothoracic limbs. As in Arachnidæ, however, the mouth is adapted for sucking, but the jaws are often partially united, and form, with a plate termed theepistomeand the labium, a beak. The mandibles are either pincer-like, or simply pointed at the tip, forming piercing organs; the palpi have their basal segments, or maxillæ, united, which form a conspicuous plate, orhypostomes, constituting the floor of the mouth. These organs are often seen to be separated from the rest of the cephalothorax by a membranous joint, and constitute a kind of head, thecapitulum. In most cases no trace of special respiratory organs can be found. Another characteristic of value in separating ticks from harvest-spiders is that in the former the young undergo a metamorphosis in the course of growth, being hatched as six-footed larvæ, and acquiring later in life a fourth pair of legs.
Fig. 412.A.Atax spinipes, water mite seen from below;B.Water Scorpion infested by Atax.
Fig. 412.
A.Atax spinipes, water mite seen from below;B.Water Scorpion infested by Atax.
The Acariæ include a number of families, all distinguished by the position of the respiratory stigmata and the form of the mandibles and palpi. In the velvety mites (Trombidiidæ), the integument is soft and covered with variously-coloured fine hairs, and the legs areadapted for walking, running or swimming. The latter live in fresh-water ponds, creeping over the leaves of aquatic plants. The fresh-water mites (Atax spinipes,Fig. 412) swim about freely by means of vigorous strokes of their legs, which act as oars. In the adult the body is more or less spherical, and usually of a bright red or greenish colour. The males of one species have a curious blunt tail-like prolongation from the hinder end of the abdomen. The eggs are laid in the spring on the stems of water plants, and the six-footed larvæ when hatched attach themselves to water-bugs (Nepa) or water-beetles (Dytiscus) by means of a large sucker developed on the front of the head.
Fig. 413.—Ixodes ricinusor Sheep-tick (under surface). The small circle encloses one life-size.
Fig. 413.—Ixodes ricinusor Sheep-tick (under surface). The small circle encloses one life-size.
Of all the Acari, the best known and most troublesome are those belonging to the family Ixodidæ; these infest the whole animal creation. They are furnished with a long cylindrical beak, armed with recurved hooks, formed of the two mandibles above and the long slender labium below. They have no eyes, nor apparently any dermaploptic sense, but there are various seemingly sensitive setædistributed over the body and on the appendages. The whole of the mites will be found suitable objects for the study of development, as the process is slow and their eggs do not require much care. The segmentation of the eggs differs; some of the cells are distinguished by their large nuclei, which stain feebly by carmine. During the cleavage of the egg no division of the so-called yolk has been observed, but later on this breaks up into several minute pieces.
Fig 414.—Mouth organs of Sheep-tick.c.Capitulum;d, e, f, g.Segments of palpi;h.Labial process;i.Spiny beak formed of fused mandibles.—(Warne.)
Fig 414.—Mouth organs of Sheep-tick.
c.Capitulum;d, e, f, g.Segments of palpi;h.Labial process;i.Spiny beak formed of fused mandibles.—(Warne.)
The accompanyingFig. 413shows the under surface of the body and the mouth parts of the common English dog and sheep tick,Ixodes ricinus, with its six formidable legs. The upper surface is shown inFig. 415; the head (capitulum) and mouth organs inFig. 414,c,d,e,f,g, together with the four segments of the palpi;hthe labial process armed with hooks forming the lower side of the beak, andiindicating the tips of the two mandibles forming the upper side, and projecting beyond the apex of the labium. By means of this beak, which is thrust to its base into the integument, the tick adheres firmly to its host, and in detaching them care must be taken that the head is not left behind buried in the skin. This tick is found in all stages of growth; the females, gorging themselves with blood, swell up to the size of a pea, as seen inFig. 413, but the male, formerly regarded as a distinct species, is of a much smaller size. In distribution these pests are almost cosmopolitan, and in tropical countries they grow to much greater dimensions, the females sometimes attaining the size of a large gooseberry.
The family of true mites is that of the Sarcoptidæ; these are either free or parasitic. They have no breathing organs; the palpiare basally fused to the rostrum, the mandibles are pincer-like, and the tarsi are often furnished at their tips with a sucker. The most familiar is the cheese mite, Tyroglyphus, which feeds upon decaying matter.
Fig. 415.1. Female Sheep-tick; 2. Rat-tick; 3. Head of Cat-flea; 4. Larva of Flea. (The life size is given in circles.)
Fig. 415.
1. Female Sheep-tick; 2. Rat-tick; 3. Head of Cat-flea; 4. Larva of Flea. (The life size is given in circles.)
The well-known cheese mite attains to a size plainly visible to the naked eye, but when first hatched out from the egg (shown in its several stages of development inFig. 417), requires a moderate amount of magnification. Its growth, however, is rapidand the young begin to feed as soon as they leave the egg. The body is partially covered over by setæ, or hairs, and the feet terminate in hooklets, as seen in the full-grown acarus. Themandibles are cutting, but as a rule they prefer soft and partially-decayed kinds of food. It also feeds upon damaged flour, sugar, and other domestic articles. TheDermestes lardarius, one of the minute beetle tribe (Fig. 418), commits even greater depredations among insect and other collections during the larval stage of its existence.
Fig. 416.Tyroglyphus. 1.Pediculus vulgaris× 50 diameters; 2.Acarus destructorunder surface; 3.Sarcoptes scabici, Itch-insect, magnified 350 diameters; 4.Demodex folliculorumfrom the human skin in various stages of growth, from the egg upwards, magnified 400 diameters. (The small circles enclose the objects of the natural size.)
Fig. 416.
Tyroglyphus. 1.Pediculus vulgaris× 50 diameters; 2.Acarus destructorunder surface; 3.Sarcoptes scabici, Itch-insect, magnified 350 diameters; 4.Demodex folliculorumfrom the human skin in various stages of growth, from the egg upwards, magnified 400 diameters. (The small circles enclose the objects of the natural size.)
Fig. 417.—The Cheese Mite,Acarus domesticus, seen in its several stages of development.
Fig. 417.—The Cheese Mite,Acarus domesticus, seen in its several stages of development.
Fig. 418.—Dermestes lardarius: larva, pupa, and imago. (Natural size.)
Fig. 418.—Dermestes lardarius: larva, pupa, and imago. (Natural size.)
Birds suffer much from mites living parasitically upon them belonging to Sarcoptidæ; these likewise infest mankind, and give rise to a disease known as the itch (Fig. 416, No. 3). This malady and the irritation accompanying it are caused by the mite excavating tunnels under the skin. In these the eggs are laid and hatched, and the young then start burrowing on their own account; their burrows are traced as whitish lines on the surface of the skin.
Fig. 416, No. 4,Demodex folliculorum, is another remarkable parasite found beneath the skin; this is usually obtained from a spot where the sebaceous follicles or fat glands are abundant, such as the forehead, the side of the nose, and the angles between the nose and lip. If the part where a little black spot or a pustule is seen be squeezed rather hard, the oily matter there accumulated will be forced out in aglobular form. This minute mite is less than one-fiftieth of an inch in length; if it be laid on a glass slide, and a small quantity of glycerine added to cause the separation of the harder portions, the parasite in all probability will float out, and, by means of a fine-pointed pencil or brush, can be transferred to a clean slide and mounted in Canada balsam. An allied species is found in the skin of dogs suffering from mange.
Fig. 419.1. Parasite of Turkey; 2. Acarus of common Fowl, under surface; 3. Parasite of Pheasant. (The small circles enclose each about life size.)
Fig. 419.
1. Parasite of Turkey; 2. Acarus of common Fowl, under surface; 3. Parasite of Pheasant. (The small circles enclose each about life size.)
The Stylopidæ are remarkable parasites, living upon the bodiesof wasps, bees, and bugs, and present a type of structure quite distinct from beetles or the ticks described. The male (Xenos peckii,Fig. 420) is a winged insect with coarsely faceted eyes, large fan-shaped wings, extremely small inconspicuous elytra, the two first thoracic rings short, while the metathorax is elongated and covers the base of the abdomen, and the hind legs are placed a long way behind the middle pair. The female, on the other hand, is a grub-like creature, without legs, wings, or eyes; she never leaves the body of her host, and from her eggs active little larvæ develop and get carried into the nests of bees and wasps.
Fig. 420.—Xenos peckii.1. Male; 2. Female.
Fig. 420.—Xenos peckii.1. Male; 2. Female.
Mites are very numerous, differ in form, and are interesting objects under the microscope. The body of the common flea (Fig. 421) is divided into distinct segments, those about the thorax being separated. Although apterous, the flea has the rudiments of four wings in the form of horny plates on both sides of the thoracic segments. Its mouth consists of a pair of sword-shaped mandibles, finely serrated; these, with a sharp, penetrating, needle-like organ, constitute the formidable weapons with which it pierces through the skin.
The neck is distinctly separated, and the body covered with scales, the edges of which are beset with short setæ; from the head project a short pair of antennæ, below which are a proboscis and a lance-shaped cutting apparatus. On each side of the head a large compound eye is placed; it has six many-jointed powerful legs, terminating in two-hooked claws; a pair of long hind legs are kept folded up when the insect is at rest, which, in the act of jumping, it suddenly straightens out with great muscular force. The female flea (Fig. 421) lays a great number of eggs, sticking them together with a glutinous secretion; the flea infesting the dog or cat glues its eggs to the roots of the hairs. In about four days the eggs are hatched out, and a small white larva or grub is seen crawling about, andfeeding most actively.Plate VI., No. 141, is a magnified view of one covered with short hairs. After nine or ten days the larva assumes the pupa form; this it retains four days, and in nine days more it becomes a perfect flea. The head of the flea found in the cat (Fig. 415, No. 3) somewhat differs in form from that of the species infesting the human being; its jaws are furnished with more formidable-lookingmandibles, and from between the first and second joints behind the head short strong spines project.
Fig. 421.1. Female Flea; 2. Male Flea. (The small circles enclose fleas of about life size.)
Fig. 421.
1. Female Flea; 2. Male Flea. (The small circles enclose fleas of about life size.)
Fig. 422.1. Parasite of Eagle; 2. Parasite of Vulture; 3. Parasite of Pigeon,Sarcoptes palumbinus.(The circles enclose each about life size.)
Fig. 422.
1. Parasite of Eagle; 2. Parasite of Vulture; 3. Parasite of Pigeon,Sarcoptes palumbinus.(The circles enclose each about life size.)
Two small and obscure groups of the mites and ticks have been associated with the latter, but for no better reason than that their affinities are unknown. The first of these are the Tardigrada, or bear animalcules, which comprise microscopical animals living in damp, sandy, and mossy places; the body is long and oval in shape, and possesses four pairs of bud-like unjointed appendages, each tipped with claws: the last pair of legs project from the hinder part of thebody. The mouth is much subdued, and only a trace of jaws is found as a pair of stylets; there appear to be no organs of respiration or circulation, and, unlike what obtains in all true Arachnida, the sexes are united in each individual. These curious infusorial creatures have been found by myself in an infusion of cow manure.
Injurious Insects.—In describing some of the more interesting points in connection with insect life, I have only quite incidentally referred to the destructive habits of the larger number of insects and the ravages annually inflicted, chiefly by the smaller parasitical tribes, upon our cultivated crops of all kinds.
Here we have a wide field of research open to the microscopist, whose investigations must be carried out systematically, day by day, and for which a moderate power will effectually serve his purpose.
There are some ten or twelve species of injurious insects that attack the hop plant. By way of example, I will select one of the least known among them, the hop-flea, or beetle (Haltica concinna). This is sufficiently minute to require the aid of the microscope, and very closely resembles the turnip-flea proper,H. nemorum. Under the microscope the former will be seen to differ considerably. Its colour is brassy, whereas the colour of its congener is dusky or black, and its wing-cases are striped. They both have wonderful powers of jumping.H. concinnahas a curious toothed formation of the tibia, with a set of spines, while the tibia of the turnip-flea is without any curve. It presents other points of difference. The hop-flea is, in fact, a winged beetle, and passes the winter in the perfect state under clods, tufts of grass, or weeds outside the hop-plantation, and here it lays its eggs. In the early spring the larvæ are hatched out as a little white maggot, which immediately makes its way to the hop-plant and burrows into the young leaves and feeds upon its tissues. Here we have an insect taken at random from among thousands of others of the most destructive kinds which annually destroy crops of enormous value to the nation.
Tuffen West, del.Edmund Evans.Plate VII.
Tuffen West, del.Edmund Evans.
Plate VII.
The most complicated condition in which matter exists is where, under the influence of life, it forms bodies with a structure of tubes and cavities in which fluids are incessantly in motion, and producing continuous changes. These have been rightly designated “organised bodies,” because of the various organs they contain. The two principal classes into which organised bodies have been divided are recognised as vegetable and animal. It was Bichat who taught that our animal life is double, while our organic life is single. In organic life, to stop is to die; and the life we have in common with vegetables never sleeps, and if the circulation of the fluids within the animal body ceases for a few seconds, it ceases for ever. In the vertebrate body, however, the combination of organs attains to the highest development, in striking contrast with that of the class we have previously considered, the Invertebrata, the animal kingdom being divided into Vertebrates and Invertebrates.
The Vertebrata are distinguished from all other animals by the circumstance that a transverse and a vertical section of the body exhibits two cavities completely separated from one another by a partition. A still more characteristic feature separates the one from the other; it is the specialisation of the chief nervous centres, and their peculiar relation to the other systems of the body.
The dorsal cavity of the body contains the cerebro-spinal nervous system, the ventral, the alimentary canal, the heart, and usually a double chain of ganglia; these pass under the name of the sympathetic system. It is very probable that this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. In any case, the central parts of the cerebro-spinal nervous system—i.e., the brain and thespinal cord—would appear to be unrepresented among invertebrate animals. Likewise, in the partition between the cerebro-spinal and visceral tubes, certain structures which are not represented in Invertebrates are contained. During the embryonic condition of all Vertebrates, the centre of the partition is occupied by an elongated cellular cylindrical mass, the notochord, or chorda dorsalis. This structure persists throughout the life in some Vertebrata, but in most it is more or less completely replaced by a jointed, partly fibrous, cartilaginous, and bony vertical column. All vertebrate animals have a complete vascular system. In the thorax and abdomen, in place of a single perivisceral cavity, in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior, recalling in the latter case the atrial cavities of the Mollusca. In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess a hepatic portal system, the blood of the alimentary canal never being wholly returned directly to the heart by the ordinary veins, but being more or less completely collected into a trunk (the portal vein), which ramifies through and supplies the liver.
With reference to one other point of importance, the development of the ova of Vertebrates, these have the same primary composition as those of other animals, consisting of a germinal vesicle containing one or more germinal nuclei, and included within a vitellus. But as this forms a part of general anatomy, and as my object is simply the investigation of the fundamental and microscopical structure of animal organisms, I shall not further pursue the morphological part of the subject, especially as so many excellent text-books are within reach of the student who desires to fully acquaint himself with precise information.
Notwithstanding, then, the apparent diversity in the structure of the vertebrate and the invertebrate and the various tissues of which animals and vegetables are constituted, microscopical research has satisfactorily demonstrated that all textures have their origin in cells; in fact, when the formative process is complete, the animal cell is seen to consist of the same parts and almost the same chemical constituents as the typical cell of the plant—namely, a definite cell-wall enclosing cell contents, of which the nature may be diverse, butthe cell nucleus is precisely the same and is the actual seat and origin of all formative activity. The cell and nucleus grow by assimilation or intersusception, that is, by inflowing of nutrition among all parts, the new replacing the old, yet maintaining its original structure and composition. That which was once thought special to animals is now found to be common to both plants and animals: they are found to be alike fundamentally in internal structure, and in the discharge of the mysterious processes of reproduction and of nutrition, although the latter forms a convenient line of separation. Life in plants goes on indefinitely; cuttings may be taken without injury to their vigour and duration of life. The same may be said of some of the lower forms of invertebrate life; for example, the hydra, the anemone, and some other well-known animals, may be cut up, divided into several parts, each one of which will form a new animal, provided a nucleus be included in the section. Nevertheless, the organisation of the amœba and the hydra is as complete for its purpose as that of man for his, and the evidence of continuity forbids the drawing of hard and fast lines, as was formerly done between the two kingdoms, the animal and vegetable. The amount of similarity or agreement in the organisation of animals is various. Animals indeed differ from each other in slight points only, for the discovery of which the microscope must be brought into requisition. Living matter in its earliest stage and simplest form appears to the naked eye as a homogeneous structure, but when placed under the highest powers of the microscope, it is seen not to be so.
But perhaps the most marked feature of the age has been the increasing attention given to the study of the lower forms of life, using their simpler structures and more diffuse phenomena to elucidate the more general properties of living matter. To understand life we must understand protoplasm. Of this there can be no doubt, as we have seen in a previous chapter that a whole family, the Monera, consists of this simple living, microscopic, jelly-like substance, which has not even begun to be differentiated, as in the amœba, which has as yet no special organs, and every speck becomes a mouth or a stomach, and which can be turned inside out and shoot out tongues of jelly to move and feel with. “Reproduction is the faculty most characteristic of life, and sharply distinguishes the organicfrom the inorganic.” It is, then, the corpuscles of protoplasm, called cells (cellulæ), which have so much interest for the physiologist, and these, like the cytods, may form independent organisms, which are then termed unicellular. Again, cells form other cells, and a multicellular organism results, and goes on increasing in geometrical progression. In the Vertebrata the cell retains its characteristic spheroidal shape, as seen inFig. 423, and undergoes division by virtue of its living protoplasmic mass.
Fig. 423.1. Newly formed cell structure; 2. Division of the nucleus; 3. It changes its situation in the cell; 4. Subdivides and breaks up; 5. Cell-walls increase in thickness; 6. Branch out into stellate cells; 7. Two cells coalesce; 8 and 9. Become multicellular.
Fig. 423.
1. Newly formed cell structure; 2. Division of the nucleus; 3. It changes its situation in the cell; 4. Subdivides and breaks up; 5. Cell-walls increase in thickness; 6. Branch out into stellate cells; 7. Two cells coalesce; 8 and 9. Become multicellular.
Epithelial Cells.—All free surfaces of the human body, both internal and external, are to a very considerable extent covered by epithelium cells. These cells are everywhere the same, but with modifications in shape and arrangement. Epithelial cells are nucleated and always joined by their surfaces or edges, without, on the external surfaces, the intervention of connective tissue.
There are four essential varieties:—1. Tesselated; 2. Columnar; 3. Spheroidal; 4. Ciliated; in all of which the nucleus remains remarkably uniform in its characters, is either round or oval, and flattened out, measuring1⁄6000th to1⁄4000th of an inch in diameter. They are insoluble in acetic acid, colourless, or slightly tinted by the structure with which they are in contact, and usually contain one or more nucleoli with a few minute irregular granules, as represented inFig. 424.
The simplest and most commonly distributed variety is the tesselated, known also as the scaly, squamous, pavement, andflattened epithelium, always arranged in single layers, lining serous cavities, many parts of the mucous membrane, and the interior of ducts and blood vessels. Upon the external surface of the body it occurs in superimposed layers, forming the “stratified epidermis.” To obtain specimens of lamellar epithelium it is only necessary to collect a little saliva, or pass a glass slide over the lining membrane of the cheek, cover it with a thin cover glass, and examine it with a ¼-inch objective. Pavement epithelium is the elementary structure of hair, nails, and horn.
PLATE XIX.ANIMAL TISSUES.
PLATE XIX.
ANIMAL TISSUES.
Columnar epithelium exists upon the mucous membrane of the stomach, on the villi of the intestines, and in the several canals. It occupies either a vertical or horizontal position, and may be detached in rows, as shown inPlate XIX., No. 2, a section taken from the intestine of a rabbit. This variety, when more highly magnified, as inFig. 424, is seen to consist of club-shaped nucleated cells, the thicker end being turned towards the surface. The protoplasm of the cell is granular, and the presence of minute vacuoles and fatty globules occupy a great part of the space. The nucleus is now seen to contain a fine network. At times the outer end of the cell is distended, as inFig. 3. This form of columnar epithelium (known as the “goblet” cell) presents a close and remarkable resemblance to the cilio-flagellate “collared” infusorial monad in its extended “wine-glass” form.
Fig. 424.No. 1. Pavement epithelium, taken from an internal membrane; 2. Columnar epithelium, from the intestine of a rabbit, showing central fat globules, and at str a fine ciliated border; 3. A so-called “goblet”-cell.
Fig. 424.
No. 1. Pavement epithelium, taken from an internal membrane; 2. Columnar epithelium, from the intestine of a rabbit, showing central fat globules, and at str a fine ciliated border; 3. A so-called “goblet”-cell.
Spheroidal epithelium is confined to the closed cavities of the body,and in the internal structure of the ducts of secreting glands. The cells are, for the most part, circular, although some are flattened out at the sides in which they are in contact with each other (Plate XIX., No. 1a). Specimens of this form may be taken from the internal surface of one of the lower animals with a scalpel. The collected matter must be placed in a drop of distilled water and examined with a high power.
Ciliated epithelium is characterised by the presence of those fine hair-like filaments (cilia) attached to the free surface of the cell. During life, and for some time after death, the cilia are seen to retain their constant waving motion. The cilia all move in one direction and rhythmically, thus giving rise to the appearance of a succession of undulations. Ciliated epithelium is found lining the mucous membrane of the air passages and nasal ducts, and wherever it is necessary to urge on a secretion by mechanical means, ciliated epithelium exists. Specimens for examination are easily obtained from the oyster, and with care will show the characteristic motion. A portion of a gill separated from the mollusc will live on for a considerable time if kept in a little of its natural secretion. The parameciæ, rotifera, and all the ciliata, are furnished with cilia as a means of locomotion and obtaining sustenance. By snipping off a small piece from the gills of the mussel, always accessible to the microscopist, and covering it over with thin glass to prevent evaporation of the animal juices, its cilia will continue to work for hours.
Lymph and Blood,Fig. 425B,a a.—There are other cells in the animal body which possess a certain amount of resemblance to those confined to the more superficial structures—i.e., the lymph, chyle, and blood. These fluids present in one respect a physical uniformity of composition, and a resemblance in the size of their characteristic corpuscles. Chyle contains besides the corpuscles of lymph, a quantity of minute granules which imparts a white colour to the fluid. Intermixed are oil globules, free nuclei, and sometimes a few red blood discs. Chyle may be had for microscopic examination by squeezing a little juice from the lymphatic gland of a sheep just slaughtered.
Fig. 425.—Human Blood Corpuscles and Crystals.A.a a.Red blood corpuscles lying flat on the warm stage;b b.in profile;c c.arranged in rouleaux;d. crenated;e. rendered spherical by water;I.leucocytes and white amœboid corpuscles;B.Blood discs of fowl, red and white, others seen in convexity and with a nucleus. Blood Crystals.—C.Hæmatin from human blood;D.Hæmatoidin;E.Hæmin;F.Tetrahedral;G.Pentagonal;H.Octahedral crystals from blood of mouse.
Fig. 425.—Human Blood Corpuscles and Crystals.
A.a a.Red blood corpuscles lying flat on the warm stage;b b.in profile;c c.arranged in rouleaux;d. crenated;e. rendered spherical by water;I.leucocytes and white amœboid corpuscles;B.Blood discs of fowl, red and white, others seen in convexity and with a nucleus. Blood Crystals.—C.Hæmatin from human blood;D.Hæmatoidin;E.Hæmin;F.Tetrahedral;G.Pentagonal;H.Octahedral crystals from blood of mouse.
Blood Corpusclesor cells vary considerably in mammals, birds, reptiles, and fishes.Fig. 102(page 143) is a microphotograph of a drop of blood magnified 3,500 times; andFig. 425, A, shows both red and whitediscs drawn to scale, magnified 1,200 diameters. The red corpuscles of human blood are distinguished by their clearly defined outlines and dark centres. Each disc is biconcave in form, and hence the whole surface cannot be focussed at the same time. When the circumference is well illuminated the centre is dark, but by bringing the objective nearer to the object, the concavity of the disc is brought into focus. It generally happens that blood corpuscles, on being first drawn, run together, and present the appearance of rolls of coins; or they may be scattered about over the field. There is a considerable difference in the form of the discs; they are circular in all mammals,except the camel, dromedary, and llama, these being oval. In profile blood corpuscles are biconcave, their investing membrane is homogeneous and elastic, and will readily move along the smallest capillary vessels. There is no trace of a nucleus in the blood-discs of the adult Mammalia, while in size they bear no proportion to the bulk of the animal in whose blood-vessels they circulate. The corpuscles of Mammalia in general are like those of man in form and size, being either a little larger or smaller. The most marked exception is the blood of the musk-deer, in which the corpuscles are of extreme smallness, about the1⁄12000th of an inch in diameter. In the elephant they are large, about1⁄2700th of an inch in diameter. The goat, among common animals, has very small corpuscles, but they are, withal, twice as large as those of the musk-deer. In theMenobranchus lateralisthey are of a much larger size than in any animal, being the1⁄350th of an inch; in the proteus, the1⁄400th of an inch in the longest diameter; in the salamander, or water-newt,1⁄600th; in the frog,1⁄900th; lizards,1⁄1400th; in birds,1⁄1700th; and in man,1⁄3200th of an inch. Of fishes, the cartilaginous have the largest corpuscles; in gold-fish, they are about the1⁄1700th of an inch in their longest diameter.
The large size of the blood discs in reptiles, especially in the Batrachia, has been of great service to physiologists by enabling them to ascertain many particulars regarding structure which could not have been otherwise determined with certainty. The value of the spectroscope in the chemical examination of the blood has been already referred to. See page 252.
White corpuscles or leucocytes (Fig. 425, I) differ materially from the red. They are large, spheroidal, finely granular masses of about1⁄2800th of an inch in diameter. In a cubic millimètre of human blood there are about 10,000 white corpuscles. They have a lower specific gravity than the red, have no cell-wall, and their substance mainly consists of protoplasm. The internal granular appearance is now believed to be due to a fine intercellular network having small dots at the intersections of the web. In the meshes of the net a hyaline substance is interspersed. They possess one or more nuclei; these are seen on the application of a few drops of acetic acid. When examined in a perfectly fresh state, especially if the glass slide be placed on the warm stage of the microscope, they exhibit a spontaneouschange of shape, amœba-like, such movements being accordingly termed amœboid. The movements referred to consist in the protrusion of processes of protoplasm which are retracted and other processes protruded as represented (Fig. 425, I). Both in human blood and in newts there are colourless corpuscles which contain coarser granules than others; these are called granular corpuscles. Some are shown near the amœboid bodies. The white corpuscles are readily found in various tissues of the body, as in the lymphatic glands. In inflammatory diseases theseleucocytespass through the walls of the capillaries into the tissues, and form morbid products, pus-cells.
Sections of blood discs are made by dipping a fine needle in a drop of blood as it exudes from a prick of the finger and drawing thin lines across the glass slip, allowing time to dry, and then cutting the lines across in all directions with a razor. The loosened portions should be removed with a camel’s-hair brush.
In birds, the blood discs are oval in shape and possess a nucleus, shown inFig. 425B, in the blood of the fowl; this is rendered more apparent on adding a drop of acetic acid. The blood of fishes is also oval and nucleated, rather more pointed than that of birds. In reptiles generally the red blood discs are large, oval, nucleated bodies, the white corpuscles still preserving their invariable circular form and granular appearance. In the salamander and proteus the discs attain to their greatest size. In the former they measure1⁄700th of an inch, and in the latter1⁄400th.
Blood Crystals.—In addition to the elements described, the blood contains various crystalline forms, represented inFig. 425, C to H. In connection with the micro-spectroscope (p. 253), the spectra of certain blood crystals are given; although varying in different animals, sufficient uniformity prevails as to render them characteristic. The crystals are formed when a little blood is mixed with water on the slide, allowing a short time for crystallisation. Near the edge of the cover-glass, where crystals begin to form, they are more distinct, but a high power is required for their examination. In human blood the crystals are prismatic; in that of the guinea-pig, tetrahedral; in the blood of the mouse, octahedral. Other forms may be obtained by the aid of chemical reagents.
In human blood there are at least three distinct forms of crystals:Hæmatinis formed in normal blood, is made visible on the additionof a little water to blood, or by agitation with ether, so as to dissolve the cell-wall of the blood corpuscles, and allow the contents to escape. A drop of blood will furnish crystals large enough to be seen with a moderate power.Hæmatoidincrystals are abnormal products, found in connection with certain diseased conditions. These crystals are seen as represented at D.Hæmincrystals must be regarded as artificial chemical products, the result of treating blood with glacial acetic acid; the acicular crystals at E, reddish-brown in colour, are artificially produced.
Fig. 426.1. White fibrous or non-elastic tissue; 2. Yellow fibrous elastic tissue.
Fig. 426.
1. White fibrous or non-elastic tissue; 2. Yellow fibrous elastic tissue.
Basement Membrane—Connective Tissue System.—Connective or areolar tissue is present almost throughout the whole of the human body, and serves to connect the various organs with one another, as well as to bind together the several parts. The muscles are surrounded by a connective tissue sheath; this penetrates into their substance, and binds together fasciculi and fibres. The same tissue is present in the skin and the mucous membranes; it also forms a sheath for the arteries, veins, and nerves. It is plentifully supplied by blood-vessels, and nerves pass through its substance. Microscopically, four different elements can be clearly made out:—1. Connective tissue cells or corpuscles; 2. White fibrous tissue; 3. Yellow fibrous tissue; 4. Ground substance.
On examining the connective tissue cells of young animals, variouscells will be seen with fine granular contents, together with nuclei, lying in spaces in the ground substance, some branched, others flattened or rounded. Even tissues supposed to be homogeneous in structure, are on staining seen to have connective tissue cells, such as those represented in a section of the cornea of the eye (see p. 31). In this case the connective tissue cells are termed corneal corpuscles; the branched cells, it will be noticed, are united by branches.
The cells in the fibrous tissue of tendons are square or oblong, and form continuous rows. White fibrous tissue is distributed throughout the animal body, but in a variety of forms; it is found in the skin and other membranes, and in all parts where strength and flexibility are necessary. The structure of white and yellow fibrous tissues is shown inFigs. 426and427.