Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/380

Rh 358 MAMMALIA [SKELETON. highest known number, 46, is possessed by the African Long-tailed Manis. Connected with the under surface of the caudal vertebrae of many mammals which have the tail well de veloped are certain bones formed more or less like an inverted arch, called chevron bones, or by the French os en V. These are always situated nearly opposite to an inter- vertebral space, and are generally articulated both to the vertebra in front and the vertebra behind, but sometimes Chiefly Or entirely either FlG - 8. Anterior Surface of Fourth Caudal J. ..* Vert ebra of Porpoise (Pliocssna eommunis). s, spinous process; m, metapophysis; t, transverse process; A, chevron bone. m s to one or the other. Sternum. The sternum of mam mals is a bone, or generally a series of bones, placed longitudinally in the mesial line, on the inferior or ventral aspect of the thorax, and con nected on each side with the vertebral column by a series of more or less ossified bars called &quot;ribs.&quot; It is present in all mammals, but varies much in character in the dif ferent groups. It usually consists of a series of distinct segments placed one before the other, the anterior being called the presternum or &quot;rnanubrium sterni &quot; of hu man anatomy, and the pos terior the xiphistemum, or xiphioid or ensiform process, while the intermediate seg ments, whatever their number, constitute the mesosternum or &quot;body.&quot; In the Whale bone Whales the presternum alone is developed, and but a FiG 9 __ IIuman stcrnum and Stcnuil Single pair of Tibs IS attached Ribs, ps, presternum; ms, meso- t -A. sternum; xs, xiphistemum; c, point Ribs. The ribs form a Series Of the cartilaginous sternal ribs. long, narrow, and more or less flattened bones, extending laterally from the sides of the vertebral column, curving downwards towards the median line of the body below, and mostly joining the sides of the sternum. The posterior ribs, however, do not directly articulate with that bone, but are either attached by their extremities to the edges of each rib in front of them, and thus only indirectly join the sternum, or else they are quite free below, meeting. no part of the skeleton. These differences have given rise to the division into &quot; true &quot; and &quot; false &quot; ribs (by no means good expressions), signifying those that join the sternum directly and those that do not; and of the latter, those that are free below are called &quot; floating &quot; ribs. The portion of each rib nearest the vertebral column and that nearest the sternum differ in their characters, the latter being usually but imperfectly ossified, or remaining permanently cartilaginous. These are called &quot;costal cartilages,&quot; or when ossified &quot;sternal ribs.&quot; In the anterior part of the thorax the vertebral extremity of each rib is divided into two parts, &quot;head&quot; and &quot; tubercle&quot;; the former is attached to the side of the body of the vertebra, the latter to its transverse process; the former attachment corresponds to the interspace between the vertebrae, the head of the rib commonly articulating partly with the hinder edge of the body of the vertebra antecedent to that which bears its tubercle. Hence tho body of the last cervical vertebra usually supports part of the head of the first rib. In the posterior part of the series the capitular and tubercular at tachments com monly coalesce, and the rib is at tached solely to its correspond ing vertebra. The number of pairs of ribs is of course the same as that of F - 10. Sternum and strongly ossified Sternal Ribs of, i , i. Great Armadillo (Priodon yiyas). ps, presternum; xs, the thoracic ver- xiphistemum. tebrae. The appendicular portion of ths framework consists, An when completely developed, of two pairs of limbs, anterior dici and posterior. The anterior limb is present and fully developed in all Ant mammals, being composed of a shoulder girdle and three lim segments belonging to the limb proper, viz., the upper arm or brachium, the fore-arm or antibrachium, and the hand or manus. The shoulder girdle in the large majority of mammals is Sho in a rudimentary or rather modified condition, compared S irtl to that in which it exists in other vertebrates. In the Monotremata (Ornithorhynchus and Echidna) alone is the ventral portion, or coracoid, complete and articulates with the sternum below, as in the Sauropsida. In all other mammals this portion, though ossified from a distinct centre, forms only a process, sometimes a scarcely distinct tubercle, projecting from the anterior border of the glenoid cavity of the scapula. The last-named bone is always well developed, generally broad and flat (whence its vernacular name &quot; blade bone &quot;), with a ridge called the &quot; spine &quot; on its outer surface, generally ending in a free curved process, the &quot; acromion.&quot; As the scapula affords attachment to many of the muscles which act upon the anterior limb, its form and the development of its processes are greatly modified according to the uses to which the member is put. It is most reduced and simple in character in those animals whose limbs are mere organs of support, as the Ungulates, and most complex when they are also used for grasping, climbing, or digging. The development or absence of the clavicle or &quot; collar-bone,&quot; an accessory bar which connects the sternum with the scapula and steadies the shoulder- joint, has a somewhat similar relation, though its complete absence in the Bears shows that this is not an invariable rule. A complete clavicle is found in Man and all the Primates, in Chiro})tera, all Insectivora (except Potamogale), in many Rodents, in most Edentates, and in all Marsu pials, except Perameles. More or less rudimentary clavicles (generally suspended freely in the muscles) are found in the Cat, Dog, and most Carnivora, Myrmecophaga, and some Rodents. Clavicles are altogether absent in most of the Ursidae, all the Pinnipedia, Manis among Edentates, the Cetacea, Sirenia, Proboscidea, all Ungulates, and some Rodents. The proximal segment of the limb proper contains a single bone, the humerus, and the second segment two
 * &quot; of attachment of clavicle; 1 to 10,