326 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



accordance with this the column broadens out below, especially 

 in the sacrum which transmits the weight to the innominate 

 bones. Strength and flexibility are both given by the series of 

 gentle curves into which the spine is thrown, and a kind of spring 

 is thus constituted by which the transmission of shocks to the 

 brain is prevented. The convexities of these curves are ventral 

 in the neck and loins. The vertebrae are usually thirty-three 

 in number, as follows : Cervical 7, dorsal ( = thoracic) 1 2, 

 lumbar 5, sacral 5, coccygeal ( = caudal) 4. The sacral vertebra 

 are fused into a large, strong sacrum. The caudal (coccygeal) 

 vertebrae are very much reduced, and, in the adult, generally 

 become anchylosed together into a small triangular coccyx. 



The sternum is relatively broad and flat, nor is it divided into 

 distinct sternebree, although manubrium and xiphisternum com- 

 monly remain distinct. The former does not possess a ventral keel. 



There are twelve pairs of ribs, of which the first seven are 

 " true," their costal cartilages ( = sternal ribs) uniting directly 

 with the sternum. Of the remaining five pairs of " false " ribs, 

 the first three pairs have costal cartilages connected with the 

 preceding ones, while the last two pairs (" floating " ribs) possess 

 free ventral ends. 



II. Appendicular Skeleton (1) Fore -Limb. The most note- 

 worthy point regarding the shoulder girdle is the presence of a 

 strong /-shaped clavicle or collar-bone stretching from sternum 

 to scapula. The presence of clavicles is associated with the 

 power of free lateral movement possessed by the arms, these 

 bones acting as props to keep the shoulders well apart, and 

 giving points of attachment to several muscles. Free Limb. The 

 articulation of the humerus with the very shallow glenoid cavity 

 forms an exceedingly free ball-and-socket joint, contrasting with 

 the hinge-joint at the elbow. 



The radius has a very large distal end which plays the chief part 

 in the support of the hand, and it is capable of rotation upon the 

 ulna at both ends. When the palm is placed upwards (supine 

 position) radius and ulna are parallel, but if it is then turned down- 

 wards (prone position) the radius rotates on the ulna, carrying the 

 hand round with it. In the rabbit the arm is immovably fixed 

 in the prone position. The ulna is larger than the radius, and 

 its large proximal end takes a larger share in forming the elbow 

 joint. The large olecranon process prevents over-extension. 



