SKELETON.] MAMMALIA 355 j axial portion belonging to the head and trunk, and an j appendicular portion belonging to the limbs. There are also certain bones called splanchnic, being developed within the substance of some of the viscera. Such are the os cordis and os penis found in some mammals. It is characteristic of all the larger bones of the Mam malia that their ossification takes its origin from several distinct centres. One near the middle of the bone, and spreading throughout its greater portion, constitutes the diapkysis, or " shaft," in the case of the long bones. Others near the extremities, or in projecting parts, form the epiphyses, which remain distinct during growth, but ultimately coalesce with the rest of the bone. The axial skeleton consists of the skull, the vertebral Axial column (prolonged at the posterior extremity into the tail), skeleton, the sternum, and tho ribs. In the skull of adult mammals, all the bones, except the Skull. lower jaw, the auditory ossicles, and the bones of the hyoid arch, are immovably articulated together, their edges being Pa IP the modifications of teeth of mammals, two principles, often opposed to each other, which have been at work in produc ing these modifications, must be held in view : (1) the type, or ancestral form, as we generally now call it, characteristic of each group, which iu most mammals is itself derived from the still more generalized type described above ; and (2) variations which have taken place from this type, generally in accordance with special functions which the teeth are called upon to fulfil in particular cases. These variations are sometimes so great as completely to mask the primitive type, and in this way the dentition of many animals of widely different origin has come to present a remarkable superficial resemblance, as in the case of the Wombat (a Marsupial), the Aye-Aye (a Lemur), and the Rodents, or as in the case of the Thylacine and the Dog. In all these examples indications may generally be found of the true nature of the case by examining the earlier conditions of dentition; for the characters of the milk teeth or the presence of rudimentary or deciduous mem bers of the permanent set will generally indicate the route by which the specialized dentition of the adult has been derived. It is perhaps owing to the importance of the dental armature to the well-being of the animal in procuring its susten ance, and preserving its life from the attacks of enemies, that great changes appear to have taken place so readily, and with such comparative rapidity, in the form of these organs, changes often accompanied with but little modification in the general structure of the animal. Of this pro position the Aye- Aye (Chiromys) among Lemurs, the Walrus among Seals, and the Narwhal among Dolphins form striking examples ; as, in all, the superficial characters of their dentition would entirely separate them from the animals with which all other evidence (even including the mode of development of their teeth) proves their close affinity. THE SKELETON. The skeleton is a System of hard parts, forming FIG. 4. Longitudinal and Vertical Section of the Skull of a Dog (Canis familiaris), with f i i -i j , & mandible and hyoid arch, an, anterior narial aperture; AIT, maxillo-turbinal bone; ET, a irameworK wmcn Supports and protects the ethmo-turbinal; Mr, nasal; ME, ossified portion of the mesethmoid ; CE, cribriform plate of softpr organs and tinsnp<s nf trip Vinrlv Tr nnnaiefa tue ethmo-turbinal ; Fr, frontal; Pa, parietal ; IP, interparietal ; SO. supra-occipital ; ExO, 1 UbSUeS Ol^tne DOay. It Consists ex . occipital . B0< basi-occipital; Per, periotic; BS, basi-sphenoid ; Pt, pterygoid; AS, ali- 01 dense flbrOUS and cartilaginous tissues, of Which sphenoid; OS, orbito-sphenoid ; PS, prespnenoid; PI, palatine; Vo, vomer; MX, maxilla; PMx, premaxilla ; sh, stylo-hyal ; th, epi-hyal ; ch, cerato-hyal ; bh, basihyal ; th, thyro-hyal ; s, symphysis of mandible ; cp, coronoid process ; cd, condyle ; a, angle ; id, inferior dentai canal. The mandible is displaced downwards, to show its entire form ; the * indicates the part of the cranium to which the condyle is articulated. 2 portions remain through life in this state, but the greater part is transformed during the growth of the animal into bone or osseous tissue. This is characterized by a peculiar histological structure and chemical composition, being formed mainly of a gelatinous basis, strongly impregnated with salts of lime, chiefly phosphate, and disposed in a definite manner, con taining numerous minute nucleated spaces or cavities called lacunae, connected together by delicate channels or canaliculi, which radiate in all directions from the sides of the lacunas. Parts composed of bone are, next to the teeth, the most imperishable of all the organs of the body, often retaining their exact form and internal structure for in close contact, often interlocking by means of fine denti- culations projecting from one bone and fitting into corre sponding depressions of the other, and held together by the periosteum, or fibrous membrane investing the bones, pass ing directly from one to the other, permitting no motion, beyond perhaps a slight yielding to external pressure. In old animals there is a great tendency for the different bones to become actually united by the extension of ossification from one to the other, with consequent obliteration of the sutures. The cranium, thus formed of numerous originally ages after every trace of all other portions of the organize- independent ossifications, retaining through life more or tion has completely disappeared, and thus, in the case of extinct animals, affording the only means of attaining a knowledge of their characters and affinities. 1 In the Armadillos and their extinct allies alone is an ossified exoskeleton, or bony covering developed in the skin, present. In all other mammals the skeleton is com pletely internal. It may be described as consisting of an 1 See for the principal modifications of the skeleton of this class, the large and beautifully illustrated Osteographie of De Blainville, 1835- 54 ; the section devoted to this subject in Bronn s Klassen und Ord- nungen des Thier-Reichs, by Giebel, 1874-79; and An Introduction to the Osteology of the Mammalia, by W. H. Flower, 2d ed. 1876. less of their individuality, or all fused together, according to the species, the age, or even individual peculiarity, con sists of a brain case, or bony capsule for enclosing and protecting the brain, and a face for the support of the organs of sight, smell, and taste and of those concerned in seizing and masticating the food. The brain case articulates directly with the anterior cervical vertebra, by means of a pair of oval eminences, called condyles, placed on each side of the large median foramen which transmits 2 This and many of the following figures are taken from Flower s
Osteology nf the Mammalia, Macmillan, 1876.