OSTEOLOGY. 755 



stance which depends upon the greatly-increased development of the 

 encephalon, and the consequent expansion of the cranium. 



(2150.) The number of dorsal vertebra depends upon that of the 

 ribs : thus in the Bat tribe there are only eleven ; while in some of the 

 Pachydermata (as, for example, in the Elephant and Tapir) as many as 

 twenty dorsal vertebra may be counted. The lumbar and sacral verte- 

 bras will likewise be more or less numerous in different genera ; and 

 in the number of pieces composing the coccyx, or tail, there is every 

 variety, from four to five-and-forty. 



(2151.) The thorax is enclosed by ribs, which in structure, and in their 

 mode of connexion with the dorsal vertebra, resemble those of Man. 

 At its dorsal extremity each rib is articulated by its head to the bodies 

 of the vertebras, and to the intervertebral substance ; while its tubercle, 

 or the representative of the second head of the rib of a Bird, is move- 

 ably connected with the corresponding vertebral transverse process. 

 There are no sternal ribs ; but these are represented by cartilaginous 

 pieces, whereby, towards the anterior part of the thorax, each rib is 

 attached to the side of the sternum : posteriorly, however, this con- 

 nexion does not exist. The anterior ribs are therefore called true ribs, 

 and the posterior, false, or floating ribs, precisely as in the human 

 skeleton. 



(2152.) The sternum is composed of several narrow pieces, placed in 

 a line behind each other along the middle of the breast. These pieces 

 are generally consolidated : by their lateral margins they give attach- 

 ment anteriorly to the clavicles, if these bones be present, and, behind 

 these, to the costal cartilages of the true ribs. 



(2153.) From the whole arrangement of the thorax, it is evident that 

 the ribs are capable of extensive movements of elevation and depression, 

 whereby the capacity of the whole thoracic cavity may be increased or 

 diminished movements which, aided by those of the diaphragm, draw 

 in and expel the air used for respiration. 



(2154.) The anterior extremity is appended to a broad scapula, gene- 

 rally unconnected with the rest of the skeleton, except by muscular 

 attachments. In quadrupeds that use this extremity as an instrument 

 of prehension or of flight, a clavicle is interposed between the scapula 

 and the sternum ; but most frequently this element of the shoulder is 

 deficient, and even the coracoid bone, if a vestige of it remains at all, is 

 reduced to a mere appendage to the scapula, known to the human ana- 

 tomist as the coracoid process. The rest of the limb presents the arm, 

 the fore-arm, the carpus, metacarpus, and phalanges ; but these are so 

 altered in appearance in different orders, that no general description 

 will suffice, and we must therefore defer this part of our inquiry for 

 the present. 



(2155.) In the posterior extremity there is equal dissimilarity in the 

 construction of the distal portions of the limb ; but the pelvis, although 



3c2 



