Читать книгу Mammalia - Frank E. Beddard - Страница 7

The skeleton of the Mammalia consists almost solely of the endoskeleton. It is only among the Edentata that an exoskeleton of bony plates in the skin is met with. As in other Vertebrates, the skeleton is divisible into an axial portion, the skull and vertebral column, and an appendicular skeleton, that of the limbs. The bones of mammals are well ossified, and in the adult there are but few and small tracts of cartilage left.

Vertebral Column.—The vertebral column of the mammals, like that of the higher Vertebrata, consists of a number of separate and fully-ossified vertebrae.

The constitution of a vertebra upon which all the usual processes are marked is as follows:—There is first of all the body or centrum of the vertebra, a massive piece of bone shaped like a disc or a cylinder. The centra of contiguous vertebrae are separated by a certain amount of fibrous tissue forming the intervertebral disc, and the apposed surfaces of the centra are as a rule nearly flat. In this last feature, and in the important fact that the centra are ossified from three distinct centres, the anterior and posterior pieces ("epiphyses") remaining distinct for a time, even for a long time (as in the Whales), the centra in the mammals differ from those of reptiles and birds. The epiphyses are not found throughout the vertebral column of the lowly-organised Monotremata, and they do not appear to exist in the Sirenia.

Fig. 5.—Anterior surface of Human thoracic vertebra (fourth), × ⅔. az, Anterior zygapophysis; c, body or centrum; l, lamina, and p, pedicle, of the neural arch; nc, neural canal; t, transverse process. (From Flower's Osteology of the Mammalia.)	Fig. 6.—Side view of first lumbar vertebra of Dog (Canis familiaris). × ¾. a, Anapophysis; az, anterior zygapophysis; m, metapophysis; pz, posterior zygapophysis; s, spinous process; t, transverse process. (From Flower's Osteology.)

From each side of the centrum on the dorsal side arises a process of bone which meets its fellow in the middle line above, and is from there often prolonged into a spine. A canal is thus formed which lodges the spinal cord. This arch of bone is known as the neural arch, and the dorsal process of the same as the spinous process. The sides of the neural arch bear oval facets, by which successive vertebrae articulate with one another: those situated anteriorly are the anterior zygapophyses, while those on the posterior aspect of the arch are the posterior zygapophyses; these articular facets do not exist in the tail-region of many mammals, e.g. Whales.

In addition to the dorsal median spinous process of the vertebra there may be a ventral median process, arising of course from the centrum, termed the hypapophysis.

From the sides of the neural arch, or from the centrum itself, there is commonly a longer or shorter process on each side, known as the transverse process. This is sometimes formed of two distinct processes, one above the other; in such cases the upper part is called a diapophysis, the lower a parapophysis.

The neural arch may also bear other lateral processes, of which one directed forwards is the metapophysis, the other directed backwards the anapophysis.

The series of bones which constitute the vertebral column can be divided into regions. It is possible to recognise cervical, dorsal, lumbar, sacral, and caudal vertebrae. In the case of animals with only rudimentary hind-limbs, such as the Whales, there is no recognisable sacral region. The neck or cervical vertebrae are nearly always seven in number. The well-known exceptions are the Manatee, where there are six, and certain Sloths, where there are six, eight, or nine. These rare exceptions only accentuate the very remarkable constancy in number, which is very distinctive of the mammals as compared with lower Vertebrata. There are of course abnormalities, the last cervical, and sometimes the last two, assuming the characters of the ensuing dorsals, by developing a more or less complete rib. There are also recorded examples of Bradypus, in which the number of cervicals is increased to ten. The characteristics, then, of the cervical vertebrae are, in the first place, that they do not normally bear free ribs, and that there is a break as a rule between the last cervical and the first dorsal on this account. In birds, for example, the cervicals, differing in number in different families and genera, gradually approach the dorsals by the gradually lengthening ribs. The transverse processes of the vertebrae are commonly perforated by a canal for the vertebral artery, and are bifid at their extremities. In some Ungulates these vertebrae, moreover, approximate to the vertebrae of lower Vertebrata in the fact that there are ball and socket joints between the centra, instead of only the fibrous discs of the remaining vertebrae.

The first two vertebrae of the series are always very different from those which follow. The first is termed the atlas, and articulates with the skull. The most remarkable fact about this bone (shared, however, by lower Vertebrates) is that its centrum is detached from it and attached to the next vertebra, in connexion with which it will be referred to immediately. The whole bone thus gets a ring-like form, and the salient processes of other vertebrae are but little developed, with the exception of the transverse processes, which are wide and wing-like. In many Marsupials, such as the Wombat and Kangaroo, the arch of the atlas is open below, there being no centre of ossification. In others, such as Thylacinus, there is a distinct nodule of bone in this situation not concrescent with the rest of the arch.

Fig. 7.—Human atlas (young), showing development. × ¾. as, Articular surface for occiput; g, groove for first spinal nerve and vertebral artery; i a, inferior arch; t, transverse process. (From Flower's Osteology.)	Fig. 8.—Inferior surface of atlas of Dog. × ½. sn, Foramen for first spinal nerve; v, vertebrarterial canal. (From Flower's Osteology.)

Fig. 9.—Atlas of Kangaroo.... (From Parker and Haswell's Zoology.)

The second vertebra, which is known as the axis or epistropheus, is a compound structure, the anterior "odontoid process," which fits into the ring of the atlas, being in reality the detached centrum of that vertebra.^[11] It is a curious fact about that process that it has independently become spoon-shaped in two divisions of Ungulates; that it has become so seems to be shown by the fact that in the earlier types of both it has the simple peg-like form, which is the prevailing form. The cervical vertebrae are occasionally wholly (Right Whales) or partially (many Whales, Jerboa, certain Edentates) welded into a combined mass. Indications of this have even been recorded in the human subject.

Fig. 10.—Side view of axis of Dog. × ⅔. o, Odontoid process; pz, posterior zygapophysis; s, spinous process; t, transverse process; v, vertebrarterial canal. (From Flower's Osteology.)	Fig. 11.—Anterior surface of axis of Red Deer. × ⅔. o, Odontoid process; pz, posterior zygapophysis; sn, foramen for second spinal nerve. (From Flower's Osteology.)

The dorsal vertebrae vary greatly in number: nine (Hyperoodon) seems to be the lowest number existing normally; while there may be as many as nineteen, as in Centetes, or twenty-two, as in Hyrax. These vertebrae are to be defined by the fact that they carry ribs, and the first one or two lumbars are often "converted into" dorsals by the appearance of a small supernumerary rib. The spinous processes of these vertebrae are commonly long, and sometimes very long. It is only among the Glyptodons that any of these vertebrae are fused together into a mass.

The lumbar vertebrae, which follow the dorsal, vary greatly in number. There are as few as two in the whale Neobalaena, as many as seventeen in Tursiops; this group, the Cetacea, contains the extremes. Nine lumbars are found in the Lemurs Indris and Loris. As a rule the number of lumbars is to some extent dependent upon that of the dorsals. It often happens that the number of thoraco-lumbar vertebrae is constant for a given group. Thus the Artiodactyles have nineteen of these vertebrae, and the Perissodactyles as a rule twenty-three. A greater number of dorsals implies a smaller number of lumbars, and of course vice versa. The existence of a sacral region formed of a number of vertebrae fused together and supported by the pelvic girdle is characteristic of the mammals, but is not found in the Cetacea and the Sirenia, where functional hind-limbs are wanting. Strictly speaking, the sacrum is limited to the two or three vertebrae whose expanded transverse processes meet the ilia. But to these are or may be added a variable number of vertebrae withdrawn from both the lumbar and the caudal series, which unite with each other to form the massive piece of bone which constitutes the sacrum of the adult.

Fig. 12.—Lepus cuniculus. Innominate bones and sacrum, ventral aspect. acet, Acetabulum; il, ilium; isch, ischium; obt, obturator foramen; pub, pubis; sacr, sacrum; sy, symphysis. (From Parker and Haswell's Zoology.)	Fig. 13.—Anterior surface of fourth caudal vertebra of Porpoise (Phocoena communis), × ½. h, Chevron bone; m, metapophysis; s, spinous process; t, transverse process. (From Flower's Osteology.)

The caudal vertebrae complete the series. They begin in as fully developed a condition as the lumbars, with well-marked transverse processes, etc.; but they end as no more than centra, from which sometimes tiny outgrowths represent in a rudimentary way the neural arches, etc. Very often the caudal vertebrae are furnished with ventral, generally -shaped, appendages, the chevron bones or intercentra.^[12] These are particularly conspicuous in the Whales and in the Edentates. In the former group the occurrence of the first intercentrum serves to mark the separation of the caudal from the lumbar series. The number of caudals varies from three in Man—and those quite rudimentary—to nearly fifty in Manis macrura and Microgale longicaudata.

Fig. 14.—Lateral view of skull of a Dog. C.occ, Occipital condyle; F, frontal; F.inf, infra-orbital foramen; Jg, jugal; Jm, premaxilla; L, lachrymal; M, maxilla; Maud, external auditory meatus; Md, mandible; N, nasal; P, parietal; Pal, palatine; Pjt, process of squamosal; Pt, pterygoid; Sph, alisphenoid; Sq, squamosal; Sq.occ, supraoccipital; T, tympanic. (From Wiedersheim's Comparative Anatomy.)

The Skull.—The skull in the Mammalia differs from that of the lower Vertebrata in a number of important features, which will be enumerated in the following brief sketch of its structure. In the first place, the skull is a more consolidated whole than in reptiles; the number of elements entering into its formation is less, and they are on the whole more firmly welded together than in Vertebrates standing below the Mammalia in the series. Thus in the cranial region the post- and pre-frontals, the post-orbitals and the supra-orbitals have disappeared, though now and again we are reminded of their occurrence in the ancestors of the Mammalia by a separate ossification corresponding to some of the bones. Nowhere is this consolidation seen with greater clearness than in the lower jaw. That bone, or rather each half of it, is in mammals formed of one bone, the dentary (to which occasionally, as it appears, a separate mento-Meckelian ossification may be added). The angular, splenial, and all the other elements of the reptilian jaw have vanished, though the numerous points from which the mammalian dentary ossifies is a reminiscence of a former state of affairs; and here again an occasional continuance of the separation is preserved, as the case observed by Professor Albrecht of a separate supra-angular bone in a Rorqual attests. Among other reptilian bones that are not to be found in the mammalian skull are the basipterygoids, quadrato-jugal, and supratemporal. A few of these bones, however, though no longer traceable in the adult skull save in cases of what we term abnormalities, do find their representatives in the foetal skull. Professor Parker, for example, has described a supra-orbital in the embryo Hedgehog; a supratemporal also appears to be occasionally independent.

Fig. 15.—Head of a Human embryo of the fourth month. Dissected to show the auditory ossicles, tympanic ring, and Meckel's cartilage, with the hyoid and thyroid apparatus. All these parts are delineated on a larger scale than the rest of the skull. an, Tympanic ring; b.hy, basihyal element; hy, so-called hyoid bone; in, incus; md, bony mandible; ml, malleus; st, stapes; tp, tympanum; tr, trachea; I. (mk), first skeletal (mandibular) arch (Meckel's cartilage); II. second skeletal (hyoid) arch; III. third (first branchial) arch; IV. V. fourth and fifth arches (thyroid cartilage). (From Wiedersheim's Structure of Man.)

In the mode of the articulation of the lower jaw to the skull the Mammalia apparently, perhaps really, differ from other Vertebrates. In the Amphibia and Reptilia, with which groups alone any comparisons are profitable, the lower jaw articulates by means of a quadrate bone, which may be movably or firmly attached to the skull. In the mammals the articulation of the lower jaw is with the squamosal. The nature of this articulation is one of the most debated points in comparative anatomy. Seeing that Professor Kingsley^[13] in the most recent contribution to the subject quotes no less than fifty-two different views, many of which are more or less convergent, it will be obvious that in a work like the present the matter cannot be treated exhaustively. As, however, Professor Kingsley justly says that "no single bone occupies a more important position in the discussion of the origin of the Mammalia than does the quadrate," and with equal justice adds that "upon the answer given as to its fate in this group depends, in large measure, the broader problem of the phylogeny of the Mammalia," it becomes, or indeed has long been, a matter which cannot be ignored in any work dealing with the mammals. A simple view, due to the late Dr. Baur and to Professor Dollo, commends itself at first sight as meeting the case. The last-named author holds, or held, that in all the higher Vertebrates it is at least on a priori grounds likely that two such characteristically vertebrate features as the lower jaw and the chain of bones bringing the outer world into communication with the internal organ of hearing would be homologous throughout the series. He believed, therefore, that the entire chain of ossicula auditus in the mammal is equal to the columella of the reptile, since their relations are the same to the tympanum on the one hand and to the foramen ovale on the other; and that the lower jaw articulates in the same way in both. It follows, therefore, that the glenoid part of the squamosal must be the quadrate which has become ankylosed with it after the fashion of concentration in the mammalian skull that has already been referred to. The fact that occasionally the glenoid part of the squamosal is a separate bone^[14] appeared to confirm this way of looking at the matter. But the hall-mark of truth is not always simplicity; indeed the converse appears to be frequently the case. And on the whole this view does not commend itself to zoologists at present. For it must be borne in mind that the lower jaw of the mammal is not the precise equivalent of that of the reptiles. Apart from the membrane bones, which may be collectively the equivalents of the dentary of the mammal, there is the cartilaginous articular bone to be considered, which forms the connexion between the rest of the jaw and the quadrate in reptiles. Even in the Anomodontia, whose relations to the Mammalia are considered elsewhere, there is this bone. But in these reptiles the articular bone articulates not only with the quadrate, but also to a large extent with the squamosal, the quadrate shrinking in size and developing processes which give to it very much the look of either the incus or the malleus of the mammalian ear. In fact it seems on the whole to fit in with the views of the majority, as well as with a fair interpretation of the facts of embryology, to consider that the chain of ear bones in the mammal is not the equivalent of the columella of the reptile, but that the stapes of the mammal is the columella, and that the articulare is represented by the malleus and the quadrate by the incus. It is very interesting to note this entire change of function in the bones in question. Bones which in the reptile serve as a means of attachment of the lower jaw to the skull are used in the mammal to convey the waves of sound from the tympanum of the ear to the internal organ of hearing.

Another important and diagnostic feature in the mammalian skull is that the first vertebra of the vertebral column always articulates with two separate occipital condyles, which are borne by the exoccipital bones and formed mainly though not entirely by them. Certain Anomodontia form the nearest approach to the mammals in this particular. The two condyles of Amphibia are purely exoccipital in origin.

In the Mammalia, unlike what is found in lower Vertebrates (but here again the Anomodontia form at least a partial exception), the jugal arch does not connect the face with the quadrate, for, as already said, that bone does not exist, in the Sauropsidan form, in mammals. This arch passes from the squamosal to the maxillary, and has but one separate bone in addition to those two, viz. the jugal or malar.