Fig. 54. Ventral view of the shoulder-girdleandsternum of a Lizard(Loemanctus longipes) × 2. (AfterParker.)1. interclavicle.6. glenoid cavity.2. clavicle.7. sternum.3. scapula.8. xiphisternum.4. coracoid.9. sternal rib.5. precoracoidal process.
Fig. 54. Ventral view of the shoulder-girdleandsternum of a Lizard(Loemanctus longipes) × 2. (AfterParker.)1. interclavicle.6. glenoid cavity.2. clavicle.7. sternum.3. scapula.8. xiphisternum.4. coracoid.9. sternal rib.5. precoracoidal process.
The sternum is absent in Sauropterygia, Ichthyosauria,Chelonia, Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia; while it is not well known in Theromorpha and Dinosauria. In the SauropodBrontosaurus, however, two rounded bones occur near the base of the coracoids, and these probably represent ossified patches in a sternum, which was mainly cartilaginous; similar structures occur inIguanodon.
The sternum frequently remains wholly cartilaginous, especially in Lacertilia; sometimes it becomes calcified, but true ossification does not as a rule take place.
APPENDICULAR SKELETON.
The Pectoral Girdle.
The pectoral girdle is well developed in all groups of reptiles except the Ophidia, occurring even in the limbless Amphisbaenidae. It is very solid in the Theromorpha. As a rule all three cartilage bones, scapula, coracoid, and precoracoid are represented, and frequently also the membrane bones,—clavicles, and interclavicle.
The coracoids are generally flat expanded bones, which sometimes, as in Sauropterygia and Ichthyosauria, meet in a ventral symphysis; sometimes, as in Lacertilia, are united with the sides of the sternum. In Chelonia neither the coracoids nor precoracoids meet one another, but their free ends are connected by fibrocartilaginous bands. In Lacertilia the coracoids are pierced by fenestrae.
The precoracoid is generally represented, but the Theromorpha are the only reptiles in which it is separately ossified; it forms a well-marked process on the coracoid in Lacertilia (fig. 54, 5). It is absent in Ichthyosauria, and Dinosauria, and probably in Sauropterygia. In some Lacertilia and Chelonia the sternal ends of the coracoids are unossified and form epicoracoids; in some Chelonia there are also epiprecoracoids; but neither these nor the epicoracoids overlap their fellows of the opposite side as they do in arciferous Anura (see p. 185). In some Lacertilia with degenerate limbs thepectoral girdle is also much reduced, inOphisaurus apusthe ventral borders of the coracoids are widely separated.
A scapula is always present, and is generally expanded distally, but in the Chelonia the distal end is cylindrical. In the Theromorpha it has an acromial process with which the precoracoid articulates, and it is very large in Dinosauria. In the Chelonia the scapula and precoracoid are ossified continuously. Among the Pterosauria,Pteranodonhas an unique pectoral girdle; the scapula and coracoid are ankylosed and the scapula articulates with the neural spines of several ankylosed vertebrae.
Clavicles occur in some Theromorpha such asPariasaurus, and also in the Ichthyosauria, Sauropterygia, Rhynchocephalia, and most Lacertilia. They are absent in the Pterosauria, the Chamaeleons among Lacertilia, the Ophidia and the Crocodilia. They are wanting too in the Chelonia, unless the first pair of ossifications in the plastron are to be regarded as clavicles. In the Sauropterygia bones regarded as the clavicles and interclavicle are generally well developed. The unpaired ossification in the plastron of Chelonia is an interclavicle, and a representative of the same bone occurs arising from the sternum in Pterosauria. A well developedT-shaped interclavicle is found in Ichthyosauria, Rhynchocephalia, Lacertilia, and some Theromorpha, such asPariasaurus.
The Limbs.
In most reptiles there are two pairs of pentedactylate limbs provided with claws, but in nearly all Ophidia and some Lacertilia (Amphisbaena,Lialis,Anguis) the limbs have entirely disappeared. In a few Ophidia such asPythontraces of the posterior limbs occur, and inChirotesamong the Amphisbaenidae there are minute anterior limbs. The Lacertilians,Chalcides(Seps) andOphisaurus(Bipes,Pseudopus) have very small posterior limbs.
The limbs are as a rule adapted for walking, but in Ichthyosauria, Sauropterygia, Pythonomorpha and some Chelonia, they have the form of swimming paddles, the relative size of the manus and pes being increased, while that of the proximal and middle portions of the limbs is reduced. This reduction is carried to its furthest extent in the Ichthyosauria in which radius and ulna, tibia and fibula, have the form of short polygonal bones similar to those constituting the manus and pes. In the Pythonomorpha the reduction of the limb bones is not quite so marked, in the Sauropterygia it is less, and still less in the Chelonia. In the earlier Ichthyosauria too, the limb bones are not so short as they are in the later forms. The Ichthyosaurian limb is also remarkable, firstly for the fact that both humerus and femur are terminated by concave articulating surfaces instead of by convex condyles, and secondly for the great multiplication of the phalangeal bones, each digit being sometimes composed of a series of over twenty. Sometimes too the number of series is increased, either by the bifurcation of some of the digits or by the development of marginal bones. In the Sauropterygia the phalanges are likewise increased above the normal but not so much as in Ichthyosauria. The humerus and femur of Sauropterygia are noticeable for the enormous size of the terminal epiphyses which form in each case by far the greater part of the bone.
The Anterior Limb.
The anterior limb is usually approximately equal in length to the posterior, but in many Dinosauria it is considerably the shorter of the two. The humerus is generally without distinct condyles, but they are well developed in the Theromorpha, the Lacertilia andSphenodon.
In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such asMesosaurus, the humerus has an ent-epicondylar foramen; in Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar foramen or groove;Sphenodonpossesses both ent- and ect-epicondylar foramina. The radius and ulna are always separate. In some Chelonia, such asChelydra, the carpus has a very simple arrangement, namely, a proximal row of three bones, the radiale, intermedium and ulnare, and a distal row of five carpalia, with one bone, the centrale, between the two rows. Many reptiles have a carpus only slightly different from this. Thus the carpus inSphenodondiffers mainly in having two centralia, that of most Lacertilia, in having the centrale and intermedium fused.
Crocodiles have a much reduced carpus with the radiale and ulnare considerably elongated. The manus in Chamaeleons is curiously modified, having the first three digits arranged in one group and turned inwards, and the fourth and fifth in another group turned outwards; carpalia 3 and 4 are united.
In the Pterosauria the anterior limbs form wings, the phalanges of the fifth digit being very greatly elongated to support the wing membrane. The first digit is vestigial and the second, third, and fourth are clawed.
The Pelvic Girdle.
The pelvic girdle is well developed in all reptiles which have posterior limbs, but is absent or quite vestigial in Ophidia and those Lacertilia which have no posterior limbs. The ilium and ischium agree in their general characters throughout all the various groups of reptiles, but that is not the case with the pubis.
In many reptiles such as Chelonia, Ichthyosauria and Lacertilia the ilia are small, more or less cylindrical bones either directed backwards, or vertically placed as in the Chamaeleons. In the Crocodilia they are larger and more expanded, while in Dinosauria and Pterosauria they are greatly elongated both in front of, and behind, the acetabulum. The ischia are generally strongly developed somewhat square bones meeting in a ventral symphysis. In Dinosauria the ischium (fig. 35, 9) is a much elongated and backwardly-directed bone,bearing a forwardly projecting obturator process. In Pterosauria the ischium is fused with the ilium, and in both pterosaurs and crocodiles the ilium and ischium are the only bones taking part in the formation of the acetabulum. In most Lacertilia there is an unpaired structure, thehypo-ischiumoros cloacaeprojecting back from the symphysis ischii, which is usually separated from the symphysis pubis by a large space, theforamen cordiforme. In some Lacertilia and Chelonia there is a cartilaginous bar dividing the foramen cordiforme into two obturator foramina; in many Chelonia this bar is ossified. AmongOphidia,Python,Tortrix,Typhlopsand their allies have a structure representing a vestigial ischio-pubis: but in most Ophidia there is no trace of the pelvis. In some Theromorpha all the bones of the pelvis are completely fused, forming an os innominatum as in mammals; the pubes and ischia are so completely fused that sometimes as inPariasauruseven the obturator foramina are closed.
Concerning the reptilian pubis there are considerable difficulties. Sometimes there is only a single pubic structure present, sometimes there are two. The reptilian pubis is best understood by comparing the arrangements met with in the various other groups with that in the Orthopod Dinosaurs such asIguanodon. InIguanodonthe pubis consists of two portions, viz. of a moderately broad pre-pubis directed downwards and forwards, and of a narrow greatly elongated post-pubis directed backwards parallel to the ischium. The pubis is united to both ilium and ischium, the acetabulum has a large unossified space, and neither pre-pubes nor post-pubes meet in ventral symphyses. The arrangement bears a great resemblance to that of Ratite birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria together with Theropod and Sauropod Dinosaurs the pubis corresponds to the pre-pubis ofIguanodonand is a more or less cylindrical bone expanded at both ends, meeting its fellow in a ventral symphysis. In Chelonia and Lacertilia the pubis bears a lateral process whichis homologous with the post-pubis of Iguanodon. In Lacertilia and sometimes in Chelonia there is a cartilaginous epipubis attached to the anterior border of the pubic symphysis; this is well developed in the Chamaeleons and Geckos. In Crocodilia there is, forming the anterior and ventral portion of the acetabulum, a patch of cartilage (fig. 49, 3) which is probably the true pubis homologous with that of lizards and with the pre-pubis ofIguanodon. The large bone generally called the pubis in Crocodiles is probably an epipubis.
The Posterior Limb.
The posterior limb is entirely absent in some Lacertilia and in most Ophidia, though traces occur inPython,TortrixandTyphlops. In the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs form swimming paddles and have been already referred to.
The arrangement of the proximal and middle segments of the limb is fairly constant in all reptiles with limbs adapted for walking, and the tibia and fibula are always separate. The pes is however subject to a considerable amount of variation, especially as regards the tarsus. In some Chelonia the tarsus like the carpus has an extremely simple arrangement, consisting of a proximal row of three bones, the tibiale, intermedium and fibulare, a centrale, and a distal row of five tarsalia. In most living reptiles, however, the tibiale and intermedium are as in mammals united, forming the astragalus. In Crocodiles (fig. 48, B, 15) the centrale is also united with the tibiale while the distal tarsalia are very slightly developed. The calcaneum in Crocodiles is drawn out into a long process forming a heel in a manner almost unique among Sauropsida. InSphenodonand Lacertilia the tibia and fibula articulate with a single large bone representing the whole proximal row of tarsalia.
The pes is generally pentedactylate, but in some Crocodiles the fifth digit is vestigial (fig. 48, B), and in some Dinosauria(fig. 35) there are only three digits. The North American Dinosaurs present a continuous series ranging from a pentedactylate plantigrade form likeMorosaurus, to such a form asHallopuswith a highly digitigrade and specialised pes reduced to three functional digits, and a vestigial fifth metatarsal. The second, third and fourth metatarsals in this form are nearly two-thirds as long as the femur, and the calcaneum is drawn out into a heel much as it is in most mammals.
In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to lie between the proximal and distal row of tarsals as in birds.
Birds form a large and extremely homogeneous class of the vertebrata, and are readily distinguished from all other animals by the possession of an epidermal exoskeleton having the form of feathers. Feathers differ from hairs in the fact that they grow from papillae formed of both the horny and the Malpighian layer of the epidermis, which papillae at first project from the surface, and only subsequently become imbedded in pits of the dermis. A dermal exoskeleton does not occur in birds.
The endoskeleton is characterised by its lightness, the large bones being generally hollow; but the pneumaticity does not vary in proportion to the power of flight. The cervical part of the vertebral column is very long and flexible, while the post-cervical portion is generally very rigid, owing to the fusion of many of the vertebrae, especially in the lumbar and sacral regions. The vertebrae are generally without epiphyses to their centra. The cervical vertebrae in living forms have saddle-shaped articulating surfaces, and many of them bear ribs. The thoracic ribs in almost all birds have large uncinate processes. The sternum is very large, and the ribs are always attached to its sides, not as in many reptilesto any backwardly-projecting process or processes. The sternum ossifies from two or more centres.
The skull is extremely light, and its component bones show a great tendency to fuse together completely. The facial part of the skull is prolonged into a beak, chiefly formed of the premaxillae; this beak is in all modern birds devoid of teeth, and is coated externally with a horny epidermal sheath. The quadrate is large and freely movable. The supratemporal arcade[1] is imperfect, while the infratemporal arcade[95]is complete. There are no postorbital or postfrontal bones. Neither parotic processes nor an interparietal foramen occur. There are commonly large pre-orbital vacuities. The palatines and pterygoids never form a secondary bony palate as in Crocodiles. Part of the floor of the skull is formed by a widebasitemporal(paired in the embryo) which is continued in front as a long slenderrostrum; these structures have replaced the parasphenoid of Ichthyopsids. Cartilage or bone is always developed in the sclerotic. The first branchial arch is well developed, the hyoid arch but slightly. The coracoids are large, and the clavicles are nearly always united forming thefurcula. There is no separate interclavicle and hardly any trace of a precoracoid.
The anterior limbs form wings, and the manus is in the adult always much modified, never having more than three digits. The three bones of the pelvis are, except in Archaeornithes, always ankylosed together in the adult, and the ilium is greatly prolonged in front of the acetabulum, which is perforated. The ilia are not connected with the sacrum by ossified sacral ribs. The pubes and ischia are directed backwards parallel to one another, and except in a very few forms never meet their fellows in ventral symphyses. The fibula is generally much reduced. The proximal tarsal bones are always ankylosed to the tibia, and the distal tarsals to the metatarsals, so that the ankle joint isintertarsal. The firstmetatarsal is nearly always free. The pes never has more than four digits in the adult.
The class Aves is most conveniently divided into two subclasses: 1. Archaeornithes. 2. Neornithes.
Subclass I.Archaeornithes.
The only form referred to this subclass of extinct birds isArchaeopteryx[96], the earliest known bird. In this animal the skeleton does not seem to be pneumatic. The cervical and trunk vertebrae are generally thought to be flat, certainly their articulating surfaces are not saddle-shaped. There is no long compound sacrum as in modern birds. The tail is longer than the whole body, the caudal vertebrae are twenty in number, they gradually taper as traced away from the trunk, and each bears a pair of feathers. The posterior caudal vertebrae are not united together to form apygostyle. The upper jaw bears thirteen pairs of conical teeth, planted in distinct sockets in the maxillae and premaxillae, but the mandible has only three pairs. The presence of these teeth forms the most essential difference between the skull ofArchaeopteryxand that of modern birds, and the fact that they occur on the premaxillae renders it improbable that a horny beak was present. There is a ring of ossifications in the sclerotic. The ribs do not show uncinate processes, and articulate with the vertebrae by single heads not divided into capitula and tubercula. Abdominal ribs appear to have been present. The furcula is large, and the scapula has a well developed acromion. The sternum is unknown. The radius and ulna are approximately equal in size. In the manus the first, second and third digits[97]are present, each terminated bya claw. The second digit is considerably the longest, while the third includes four phalanges. The three bones of the pelvis probably remained distinct throughout life. The tarsals are ankylosed respectively to the tibia and metatarsals as in other birds. The metatarsals are ankylosed together, and the pes has four digits.
Subclass II.Neornithes.
To this subclass may be referred all known birds exceptArchaeopteryx. They all agree in having a short tail whose component vertebrae are commonly ankylosed together forming a pygostyle. The three metacarpals do not all remain distinct. The bones of the pelvis are ankylosed together, and to a large though variable number of vertebrae. There are three orders, the Ratitae, Odontolcae, and Carinatae.
Order1.Ratitae.
The Ratitae differ fromArchaeopteryxand the great majority of Carinatae in being flightless. The bones are generally not pneumatic, containing marrow instead of air, in the Ostrich however they are very pneumatic. The tail is short and the posterior caudal vertebrae are generally ankylosed together forming a pygostyle. The pectoral girdle has comparatively a much smaller size than in Carinatae, clavicles are small or absent, and the scapula and coracoid lie nearly in the same straight line. The ilium and ischium do not as in Carinatae unite posteriorly, and enclose a foramen except in very old Rheas and Emeus. The quadrate articulates with the cranium by a single head. The vomers unite and form a broad plate, separating the palatines, pterygoids and basisphenoidal rostrum.
The anterior limbs are greatly reduced in size or even absent, while the posterior limbs are greatly developed and adapted for running. The tibia and fibula are quite distinct.
Many ornithologists agree that the various forms grouped together as Ratitae are not all very closely allied to one another, that they resemble one another mainly in having lost the power of flight, and do not form a natural group.
The Ratitae include the following groups:—
Æpyornithes[98], huge extinct birds from Madagascar.
Apteryges, including the Apteryx of New Zealand.
Dinornithes[99], the Moas, huge extinct birds from New Zealand, and some of the neighbouring islands.
Megistanes, including the Cassowaries (Casuarius) of Australia, New Guinea, and some of the neighbouring islands; and the Emeus (Dromaeus) of Australia.
Rheornithes, including the Rheas of S. America.
Struthiornithes, including the Ostriches (Struthio) now living in Africa, and found fossil in N. India and Samos.
Order2.Odontolcae.
This order includes only an extinct N. American birdHesperornis[100]. The jaws are provided with a series of sharp teeth placed in continuous grooves, but the premaxillae are toothless, and were probably sheathed in a horny beak. The rami of the mandible are not ankylosed together in front. The skeleton is not pneumatic. The cervical vertebrae have saddle-shaped articulating surfaces as in ordinary birds, and the thoracic vertebrae are not ankylosed together. The tail is comparatively long, and formed of twelve vertebrae with only slight indications of a pygostyle. The ribs have uncinate processes. The anterior limb is quite vestigial, being reduced to a slender humerus. The posterior limb is very powerful and adapted for swimming.
Order3.Carinatae.
This order includes the vast majority of living birds. The cervical vertebrae have saddle-shaped articulating surfaces (except in the Ichthyornithiformes). The posterior caudal vertebrae are ankylosed forming a pygostyle. The quadrate articulates with the cranium by a double head. In all except the Tinamidae the vomers are narrow behind and not interposed between the palatines, pterygoids and basisphenoidal rostrum. The sternum has a median keel, and the anterior limbs are in the great majority of cases adapted for flight. Clavicles are well developed, and the scapula and coracoid are nearly at right angles to one another. The various groups into which the Carinatae are divisible are shown in the table on pp. 40-42. Their special characters will not be dealt with.
Fig. 55.Gallus bankivavar.domesticus.The left half of the Skeleton.The skull, vertebral column, and sternum are bisected in the median plane. (After Marshall and Hurst.)A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle. CO, coracoid. CR, cervical rib. C 1 = one, first cervical vertebra. FE, femur. HC, ventral end of clavicle. HU, humerus. HY, hyoid. IF, ilio-sciatic foramen. IL, ilium. IS, ischium. L, lachrymal. MC 3, postaxial metacarpal. MN, mandible. MS, xiphoid processes. MT, tarso-metatarsus. MT 1, first metatarsal. N, nasal. OP, optic foramen. P, premaxillae. PB, pubis. PL, palatine. PY, pygostyle. R, radius. RC, radial carpal. S, keel of sternum. SC, scapula. T, tibio-tarsus. TH 4, fourth thoracic vertebra. U, ulna. UC, ulnar carpal. UP, uncinate process. Z, infra-orbital bar. 1, 2, 3, 4, first, second, third and fourth digits of pes. 3, pre-axial, 4, middle, and 5, postaxial digit of manus.
Fig. 55.Gallus bankivavar.domesticus.The left half of the Skeleton.The skull, vertebral column, and sternum are bisected in the median plane. (After Marshall and Hurst.)
A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle. CO, coracoid. CR, cervical rib. C 1 = one, first cervical vertebra. FE, femur. HC, ventral end of clavicle. HU, humerus. HY, hyoid. IF, ilio-sciatic foramen. IL, ilium. IS, ischium. L, lachrymal. MC 3, postaxial metacarpal. MN, mandible. MS, xiphoid processes. MT, tarso-metatarsus. MT 1, first metatarsal. N, nasal. OP, optic foramen. P, premaxillae. PB, pubis. PL, palatine. PY, pygostyle. R, radius. RC, radial carpal. S, keel of sternum. SC, scapula. T, tibio-tarsus. TH 4, fourth thoracic vertebra. U, ulna. UC, ulnar carpal. UP, uncinate process. Z, infra-orbital bar. 1, 2, 3, 4, first, second, third and fourth digits of pes. 3, pre-axial, 4, middle, and 5, postaxial digit of manus.
Fig. 55.
Fig. 55.
I. EXOSKELETON.
The exoskeleton of the Duck and indeed of all birds is entirely epidermal in origin. Its most important part consists offeathers, but it includes also the following horny structures:—
(a)scales, which cover the toes and tarso-metatarsus;
(b)claws, which are attached to the distal phalanges of the toes and of the pollex;
(c) the widebeak, which sheaths both upper and lower jaws, and whose edges are raised into lamellae, which act as strainers.
Feathers.
A well developed feather, such as one of the large quill feathers of the wing or tail, consists of the following parts: A main stem, thescapus, which forms the axis running along the whole length of the feather, and is divided into (1) a proximal hollow cylindrical portion, thecalamusorquill, and (2) a distal solid portion, therachisorshaft, which is square in section, flexible and grooved along its ventral surface, and bears a number of lateral processes, thebarbs. Thecalamuswhich is partly imbedded in a pit in the dermis,bears two holes: one, theinferior umbilicus, is at its proximal end, and into it enters a vascular outgrowth from the dermis; the other, thesuperior umbilicus, lies on the ventral surface at the junction of the calamus and scapus.
Thebarbsare a series of narrow elastic plates, attached by their bases to the rachis, and with their edges looking upwards and downwards. The barbs are connected together by a number of smaller processes, thebarbules, which interlock with one another by means of hooklets, and bear the same relation to the barbs that the barbs do to the rachis. The barbs and barbules, together with the rachis, constitute thevexillumorvaneof the feather. Any feather having the above type of structure is called apennaor acontour feather, from the fact that it helps to produce the contour of the body.
Varieties of feathers.
1.Pennae.There are two kinds of pennae or contour feathers.
(a) Thequills. These form the large feathers of the wing and tail. They are divided into two groups, theremiges, or wing quills, and therectrices, or tail quills.
Theremiges[101]include three sets of feathers, theprimariesormetacarpo-digitals, which are attached to the bones of the manus, thesecondariesorcubitals, which are attached to the ulna, and thehumerals, which are attached to the humerus.
Theprimariesdiffer from all the other quill feathers in having the posterior half of the vane much wider than the anterior half. They are ten in number, and of these six, themetacarpalquills (fig. 57, 14), are attached to the second and third metacarpals, one, thead-digital(fig. 57, 15), to the phalanx of the third digit, two, themid-digitals(fig. 57, 16), to the first phalanx of the second digit, and two, thepre-digitals(fig. 51, 17), to the second phalanx of the second digit. One of the pre-digitals is very small, and is called theremicle(fig. 57, 11).
Fig. 56. The wing of a Wild Duck(Anas boschas).The upper figure shows the dorsal side of a right wing, the lower figurethe ventral side of a left wing. × 1/3. (Brit. Mus.)1. scapulars.6. metacarpo-digitals or2. tectrices marginales.primaries.3. tectrices minores.7. tectrices mediae.4. bastard wing.8. cubitals or secondaries.5. tectrices majores.9. pennae humerales.10. pennae axillares.
Fig. 56. The wing of a Wild Duck(Anas boschas).The upper figure shows the dorsal side of a right wing, the lower figurethe ventral side of a left wing. × 1/3. (Brit. Mus.)1. scapulars.6. metacarpo-digitals or2. tectrices marginales.primaries.3. tectrices minores.7. tectrices mediae.4. bastard wing.8. cubitals or secondaries.5. tectrices majores.9. pennae humerales.10. pennae axillares.
The upper figure shows the dorsal side of a right wing, the lower figure
the ventral side of a left wing. × 1/3. (Brit. Mus.)
In addition, a group of three quill feathers is attached to the first digit, constituting thebastard wingorala spuria(fig. 56, 4).
Thesecondariesorcubitals(fig. 56, 8) form a group of seventeen feathers, attached to the ulna; they are shorter thanthe primaries, and do not have the posterior half of the vane much wider than the anterior half.
Thehumerals(figs. 56, 9 and 57, 12) form a group of eight small feathers, of varying length, attached to the anterior half of the humerus.
Fig. 57. Wings of a Wild Duck with the coverts removed(Anas boschas). × 1/3.A. Right wing seen from the dorsal side. B. Left wing disarticulatedand seen from the ventral side. (Brit. Mus.)1. humerus.10. second phalanx of second2. radius.digit3. ulna.11. remicle.4. radial carpal.12. pennae humerales.5. ulnar carpal.13. cubitals or secondaries.6. first phalanx of first digit.14. metacarpal quills.7. second metacarpal.15. ad-digital.8. third metacarpal.16. mid-digitals.9. first phalanx of second digit.17. pre-digital.
Fig. 57. Wings of a Wild Duck with the coverts removed(Anas boschas). × 1/3.A. Right wing seen from the dorsal side. B. Left wing disarticulatedand seen from the ventral side. (Brit. Mus.)1. humerus.10. second phalanx of second2. radius.digit3. ulna.11. remicle.4. radial carpal.12. pennae humerales.5. ulnar carpal.13. cubitals or secondaries.6. first phalanx of first digit.14. metacarpal quills.7. second metacarpal.15. ad-digital.8. third metacarpal.16. mid-digitals.9. first phalanx of second digit.17. pre-digital.
A. Right wing seen from the dorsal side. B. Left wing disarticulated
and seen from the ventral side. (Brit. Mus.)
(b) Thetectricesorcovertsare short feathers, which cover over the quills of the rectrices and remiges, and clothe the body generally. Their barbules are less developed than is the case with the quill feathers, so that the barbs separate readily from one another, especially at the base of the vane. The nomenclature of the various patches of coverts on the wings is seen in fig. 56. A small patch of backwardly-directed feathers surrounding the external auditory opening are known as theauriculars.
2. Thefiloplumesare rudimentary feathers, consisting of a minute stem and slightly developed vane. They are left in the skin after the other feathers have been removed.
3. Theplumulae, or down feathers, have the stem very slightly developed, while the barbs are soft and free from one another. They are distributed all over the body, not only among the contour feathers, but also over the spaces (apteria) which bear no contour feathers.
In the young bird the rudiments of the new feathers are formed at the bases of the embryonic down feathers, and as they grow they push them out from the skin. The embryonic down feathers however remain attached to the apices of the new feathers till these have reached a length of about an inch; they are then shed.
II. ENDOSKELETON.
As compared with that of the Turtle or Crocodile, the endoskeleton of the Duck is characterised by:
1. The great lightness of the bones, many of which contain air cavities.
2. The tendency to become ankylosed together shown by many of the bones.
3. The modification of the anterior limbs and girdle for the purpose of flight.
1.The Axial Skeleton.
This, as in other vertebrates, is divisible into—
A. The vertebral column. B. The skull. C. The ribs and sternum.
A.The Vertebral Column.
The vertebral column of the duck, like that of the great majority of birds, presents a number of well-marked characteristics, contrasting strongly with those of the generality of higher vertebrates. The centra are always without epiphyses. The neck is exceedingly long, about as long as all the rest of the vertebral column put together, and is remarkable for its flexibility. The trunk portion of the vertebral column on the other hand is characterised by extreme rigidity, and the marked tendency shown by the component vertebrae to fuse together into one almost continuous mass. The most rigid part of the vertebral column is that to which the pelvis is united, as no less than seventeen vertebrae take part in the union. The tail of the duck, like that of all living birds, is very short, and the posterior caudal vertebrae are unitedtogether, forming thepygostyle. The vertebral column may be divided into cervical, thoracic, lumbar, sacral, and caudal regions, but the boundaries between the several regions are ill-defined.
The Cervical Vertebrae.
All the vertebrae anterior to the first one that bears a rib meeting the sternum are regarded as cervical vertebrae. There are therefore sixteen cervical vertebrae, the last two of which bear well developed ribs. All are freely movable on one another.
As a typical cervical vertebrae, any one from the fifth to the ninth may be taken. The vertebra is rather elongated, and is very lightly and strongly made, its most characteristicfeature being the shape of the articulating surfaces of the centra, which are generally described as saddle-shaped. The anterior surface is convex from above downward, and concave from side to side, while the posterior and more prominent surface is concave from above downwards and convex from side to side. The neural arch is low, and is drawn out into a slight blade-likeneural spine. Its base is deeply notched on both sides posteriorly for the exit of the spinal nerves. Above these notches it is drawn out into two rather prominent diverging processes, which bear thepostzygapophyses,—two flattened surfaces which look downwards and outwards. Thetransverse processesform irregular outgrowths from the anterior two-thirds of the sides of the vertebra; each projects for a short distance downwards and outwards, and is terminated posteriorly by a short backwardly-projecting spine. The transverse processes are shown by development to ossify from separate centres, and are therefore to be regarded as cervical ribs, and each is perforated at its base by a canal for the passage of the vertebral artery. Above the anterior end of the vertebrarterial canal are a pair of thickened outgrowths, which bear upwardly and inwardly directedprezygapophyses. Each transverse process is perforated near its middle by a prominent foramen through which passes a vein which is connected with the jugular vein.
The third and fourth cervical vertebrae resemble the succeeding ones in most respects, but have smallhypapophyses, and the neural spines are less blade-like. The posterior cervical vertebrae (tenth to sixteenth) differ somewhat from the middle ones. They are shorter and more massive, the neural arch is much shorter, being deeply notched in the middle line in front and behind. The transverse processes arise from the anterior half of the vertebra only, and in the eleventh vertebra each is drawn out below into a pair of rather prominent downwardly and inwardly directed processes. In the twelfth vertebra these processes have almost coalesced, andin the thirteenth vertebra they have coalesced completely, forming a prominenthypapophysis. In the succeeding vertebrae this hypapophysis rapidly decreases in size.
The fifteenth and sixteenth cervical vertebrae resemble the succeeding thoracic vertebrae, having short thick centra and prominent squarely truncated neural spines; the sides of the neural arches are very deeply notched. The fifteenth vertebra has a short transverse process, perforated by a wide vertebrarterial foramen, but this foramen is absent in the sixteenth. The transverse processes of the fifteenth vertebra bear two facets for the articulation of the capitulum and tuberculum of the rib. The sixteenth vertebra has its tubercular facet on the transverse process, but the capitular facet is borne on the centrum.
The second oraxisvertebra is small, and has the centrum drawn out into a comparatively very large hypapophysis. The posterior articulating surface of the centrum is saddle-shaped, the anterior nearly flat: above it the centrum is prolonged into the prominentodontoid process, which is shown by development to be the detached centrum of the atlas. The neural arch is deeply notched in the middle line in front, and at the sides behind. It is drawn out posteriorly into a wide massive outgrowth, which overhangs the third vertebra and bears the downwardly-directed postzygapophyses. The prezygapophyses are situated at the sides of the anterior end of the neural arch, and look directly outwards. The transverse processes are very slightly developed, and are pierced by the vertebrarterial canals.
Theatlasvertebra is a very slight ring-like structure, thickened ventrally and bearing in front a prominent concave cavity for articulation with the occipital condyle of the skull. Posteriorly it bears a more or less flattened surface for articulation with the centrum of the axis. It surrounds a large cavity partially divided into a larger dorsal portion, which is the neural canal, and a smaller ventral portion which lodgesthe odontoid process. The sides of the atlas are pierced by the vertebrarterial canals, above which there are two slight backwardly-projecting outgrowths bearing the postzygapophyses on their inner faces.
The Thoracic Vertebrae.
The thoracic region includes all the vertebrae bearing free ribs, except the first two, viz. those whose ribs do not reach the sternum. There are seven thoracic vertebrae. The first four have centra with saddle-shaped articulating surfaces, but are more or less firmly united together by their neural spines; the last two are completely ankylosed by their centra to the lumbar vertebrae.
Each of the first five vertebrae has a prominent, vertical, abruptly terminated neural spine, and straight transverse processes. The zygapophyses and articulating surfaces at the ends of the centra are well developed. The third, fourth, fifth, and sixth vertebrae have very prominent hypapophyses. The articular facets for the ribs are well marked, those for the tubercula lying at the free ends of the transverse processes, and those for the capitula at the sides of the anterior ends of the centra. The sixth and seventh thoracic vertebrae are firmly fused by their centra and neural arches to one another and to the lumbar vertebrae behind, and by their transverse processes to the ilia. The sixth has its centrum terminated in front by a saddle-shaped articulating surface, and bears a pair of prominent prezygapophyses. Its transverse processes and centrum bear facets for the tubercula and capitula of the ribs respectively. In the seventh vertebra the tubercular facet is wanting.
Fig. 58. A, dorsal and B, ventral view of the pelvis and sacrum of a Duck(Anas boschas).1. ilium.4. pectineal process.2. ischium.5. lumbar vertebrae.3. pubis.6. true sacral vertebrae.
Fig. 58. A, dorsal and B, ventral view of the pelvis and sacrum of a Duck(Anas boschas).1. ilium.4. pectineal process.2. ischium.5. lumbar vertebrae.3. pubis.6. true sacral vertebrae.
The Sacrum.
Thesacrumgenerally consists of seventeen vertebrae fused with one another and with the ilia. Their number may be reckoned from the number of foramina for the exit of spinal nerves. The two most anterior of these vertebrae bear ribsand have been already described with the other thoracic vertebrae. Their neural spines and those of the four succeeding vertebrae are fused together, forming a continuous crest of bone completely united laterally with the ilia. The transverse processes of all these six vertebrae are well developed, but those of the posterior two (fig. 58, B, 5) are much the stoutest. The next three vertebrae have broad centra, but their transverse processes are very slightly developed and have no ventral elements. These seven vertebrae belong to thelumbarseries. The remaining eight vertebrae have well-developed transverse processes, which in the case of the first three or four are divisible into dorsal and ventral elements. All the dorsal elements are united to form a pair of flattened plates, partially separated by a series of foramina from amedian plate formed by the united neural arches. Laterally they are continuous with the ischia. The first two of this series of vertebrae are shown by their relation to the nerves to be the truesacrals(fig. 58, B, 6), the remaining six belonging to thecaudalseries.
Behind them come the six free caudal vertebrae, succeeded by a terminal piece, thepygostyle, formed of a number of vertebrae fused together; this bears the rectrices or tail quills.
Fig. 59. Skull of a Duck(Anas boschas). × 1.A. Dorsal view of the cranium.B. Palatal view of the mandible.C. The Hyoid.For numbers see Fig. 60.
Fig. 59. Skull of a Duck(Anas boschas). × 1.A. Dorsal view of the cranium.B. Palatal view of the mandible.C. The Hyoid.For numbers see Fig. 60.
A. Dorsal view of the cranium.B. Palatal view of the mandible.
C. The Hyoid.
For numbers see Fig. 60.
B.The Skull.