OF THE MOTOBY APPARATUS IN MAN. 135 



place at the articulation of the occipital bone and atlas ; 

 rotation of the head is performed hy the atlas and head 

 moving as one around the processus dentatus of the axis or 

 second vertebra. There exist check ligaments to prevent 

 this movement going too far. 



272. To the dorsal vertebrae are articulated the ribs, 

 24 in man, i.e., 12 on each side. The head of the rib rests 

 on the vertebral column, and the tubercle on the transverse 

 process generally ; the anterior extremity of the rib is united 

 to a cartilage, by means of which it is prolonged to the 

 sternum, directly in the first seven ribs, and indirectly in the 

 remaining five. Hence the division of the ribs into true and 

 false, or sternal and asternal (Fig. 83). 



273. Limbs. The skeleton of the limbs may be divided 

 into a basilar portion, and a lever or extended and moveable 

 part. In the pectoral extremity the basilar portion or 

 shoulder consists of two bones, the scapula and the collar- 

 bone or clavicle. With the scapula, a large and flat bone, is 

 articulated the humerus or arm bone. The articular cavity 

 (glenoid) has but little depth. On the inner side of this 

 glenoid cavity is a strong process, called coracoid, and on the 

 dorsal side of the bone a strong spine, running from near its 

 base, and terminating in a process called acromion ; sur- 

 mounting the shoulder-joints and attached to this process is 

 the clavicle, a cylindrical and slender bone, comparatively ; its 

 other extremity is articulated with the manubrium of the 

 sternum (Figs. 81 and 83). 



The more obvious use of the clavicle is to maintain the 

 shoulders apart, and hence the frequency of its fracture when 

 forced towards the sternum. These clavicles are strong in 

 birds of powerful flight, and weak in those differently circum- 

 stanced : contrast, for example, the eagle and the turkey. In 

 animals like the horse, ox, &c., they are wholly wanting ; the 

 scapula, as the essential bone of the shoulder, never. 



Numerous powerful muscles fix the shoulder-bones to the 

 trunk ; of these may be mentioned the trapezius, rhomboids, 

 and levator of the angle of the scapula ; a muscle of great 

 power, even in man, but much more so in the larger 

 quadruped mammals, as the horse, ox, &c., connects the 

 scapula with the ribs, viz., the serratus magnus. 



274. The arm is divided into arm, fore-arm, and hand. 

 In the skeleton of the arm there is one bone, the humerus ; in 

 that of the fore-arm two bones, the radius and ulna; the 



