In the armadillo the dermal bones are small, usually five or six sided, smooth internally, and variously sculptured externally—the pattern, however, being constant in, and characteristic of. each species. They are united together at their margins by rough surfaces, and collectively resemble a tesselated pavement. To allow of the requi'site movements of the trunk of the armadillo, which have the power of rolling themselves into a ball, a certain number of transverse rows are interposed, having an elastic yielding attachment with one another, and with the anterior and posterior fixed parts of the trunk-armor; and by this arrangement, the head and limbs can be withdrawn beneath the central case. by the action of strong subcutaneous muscles. In the colossal extinct armadillo (the glotodon) the trunk-armor was not divided by bands, but was composed of one immovable piece, covering the back and sides—an arrangement by which the dermo-skeleton would afford increase protection against falling timber, the attacks of other animals, etc.
The splanchno-skeleton is at first sight less apparent titan the dermo.skeleton. In most air-breathing vertebrates the larynx, trachea, and bronchial tubes contain a car tilaginous framework which sometimes becomes ossified; in fishes, and in the batra chians in the tadpole state, the gills are supported upon a cartilaginous or osseous frame work, developed independently of the vertebral skeleton; and in many mammals the heart contains a bone that serves as a support for its muscular and ligamentous fibers. If to these parts we add the so-called "sense-capsules"—the bony cap which is found in the outer coat of the eye in many birds and most fishes; the hard bony envelope which surrounds the internal ear, and which subsequently, as the petrous portion of the tem poral bone, becomes incorporated in most vertebrates with the neuro-skeleton; and the turbinate bones of the nose—and the teeth, we have the principal parts of the splanchno skeleton. The sclero-skeleton requires no further explanation than that which has been already given; and we therefore proceed to what may be called the skeleton proper—the neuro-skeleton.
From the nature of the subject it is impossible to avoid the introduction of a con siderable number of technical terms, which will probably be new, and will sound some what harshly to many of our readers; and as few writers can popularize a difficult subject more successfully than prof. Owen himself (unquestionably the greatest osteolo gist of the present age), we shall for the mostpart follow the history of the neuro skeleton, which he drew up for the benefit of general readers in The Circle of the Sciences. A thoughtful examination of the skeleton of any vertebrate shows that it is arranged in a series of segments, following and articulating with each other in the direction of the axis of the body, from before backward in brutes, from above down ward in man. Each complete segment, called a "vertebra," consists of a series of osseous pieces arranged according to the plan shown in Figs. 2 and 3, so as to form a bony hoop or arch above a central piece for the protection of a segment of the nervous axis; and a bony hoop or arch beneath the central piece for the protection of a segment of the vascular system. The upper hoop, N, is called the' neural arch" (Gr. neuron, a nerve), and the lower hoop, H, the "hemal arch" (Gr. llama, blood); while their com mon. center, C, is termed the centrum. The neural arch is formed by a pair of bones, n, called " neurapophyses" (Gr. apophysis, a projecting part or process), and by a bone, Its, sometimes cleft or bifid, called the "neural spine;" it also sometimes includes a pair of bones, d, d, called "diapophyses" (Gr. dig, across). The hemal arch is formed by a pair of bones, pl, called "pleurapophyses" (Gr, pleuron, a rib); by a second pair, h, called "hemapophyses;" and by a hone, ha, sometimes bifid, called the "hemal spine." It also sometimes includes parts or bones called "parapophyses" (Gr. para, transverse). Bones, moreover, are developed, which diverge as rays from one or more parts of a vertebra. Prof. Owen divides the various parts of a vertebra into (t) the autogenous and (2) the exogenous parts. The autogenous parts are those which are developed from independent centers of ossification (q.v.), and are termed the elements of the vertebra; while the exogenous parts are those that grow from parts previously ossified, and are termed processes. The line between those two sets of parts cannot be strictly drayn,, since parts which are usually exogenous are sometimes autogenous, and vice versa. The autogenous parts or elements are the centrum, C; the neurapophyses, v, the neural spine, ns; the pleurapophyses, pl, lit; the hemapophyses, h, h, and the hemal spine, Its; while the exogenous parts or processes are the diapophyses, the parapophyses; the zygapophyses (Fig. 2), 2, 2 (Gr. zoos, a junction); the anapophyses (Gr. ova, backward);
the metapophyses (Gr. meta, between); the hypapophysis (Gr. below); and the epa pophysis (Fig. 1), e (Gr. epi, upon). These individual parts may be united with each other in various ways, and may occur in various degrees of development; sometimes they (or some of them) remain entirely disjointed even in the adult animal, while in other cases they are united into a single piece, so that their real distinctness can only be recognized by tracing tire history of their development. In most instances some one or more of these parts will be found to be altogether deficient, while in other cases one set of parts is exaggerated to a great degree. Thus, in the third or parietal segment of the human skeleton the neural arch is much expanded, while the hemal one is contracted; while more commonly, as in the thoracic segment or vertebra of a raven, the hemal arch is much expanded and the neural one contracted; while sometimes again, as'in the tail of the crocodile and of many other animals, both neural and hemal arches are simul taneously contracted. The segments are commonly simplified and made smaller as they approach the end of the vertebral column or axis, one element or process after another being removed until the vertebra is reduced to its centrum, as in the diagram of the archetype vertebral skeleton. If we glance at the typical vertebra represented in Fig. 2, we observe the diapophyses projecting above it canal that serves for the passage of a blood-vessel, and parapophyses which form the lower boundaries of this canal. Tnese elements never attain any high development in mammals, birds, or rep tiles; thus, in the human cervical vertebra, they form the two roots of the transverse process surrounding the foramen for the passage of the vertebral artery, while in the thoracic vertebra of the bird the diapophyses form the transverse processes, and the parapophyses, reduced to mere rudiments, form the articular surfaces with which the heads of the ribs come in con tact. In fishes, however, they are much developed, and in the cod tribe are even larger and broader than the pleurapophyses or true ribs. The ordinary function of these lateral processes is to afford attachment to muscles, to protect the lateral vascular trunks (as in the case of the vertebral artery), and to give support to the pleura pophyses, pal, whose develop ment varies extremely in different parts of the seine vertebral column, as well as in different animals. Then, in the human cervical verte bra, they form the short bifid transverse processes which are an chylosed at their base to the diapophyses and parapophyses, which surround the vertebral ca nal. In the thoracic segments they are developed separately, and con stitute the ribs which form the greater part of the circumference of the hemal arch. Proceeding to the consideration of the parts be low the centrum, we often find the entire hence! arch wanting, as in the cervical and lumbar vertebrm of man and mammals; but in the tail of sonic mammals and of rep tiles a hemal arch, protecting the caudal artery and vein, and closely resembling a neural arch, is found. It is in the thoracic region of inam mals, birds, and reptiles that we find the greatest expansion of the Kemal arch; the hemapophyses here articulating with the extremi ties of the ribs instead of with the centrum, and the arch surrounding the entire visceral cavity. In man and mammals the hemapophyses remain hnossified, and are known as the cartilages of the ribs; but in birds and reptiles they are ossified, and constitute the sternal ribs. The hemal spine, hs, presents great variety of form, and is often alto gether hbsent. In the mammalian thorax it occurs as a fiat sternum; in birds the flatness is replaced by a prominent keel on the mesial line, so that a trans verse section almost resembles a neural spine; while in reptiles, again, the hemal spine or sternum is flattened laterally, as in mammals.. The hemapophyses and hems! spine are absent in the abdominal region of mammals and birds, but are continued backward in the saurians or lizard-like reptiles, whose keinal arch is, notwithstanding. incom plete, from the absence of pleurapophyses. In serpents the hemal arches are wanting through the whole trunk, the ends of the ribs being free; and in fishes generally the hemapophyses and hemal spine are absent or unossified.