STRUCTURE OF THE HUMAN BODY 29 



" chew the cud ", e.g. cow, sheep, goat, are obliged to move their 

 jaws from side to side, a kind of movement which is rendered 

 possible by flattish condyles. In human beings the lower jaw can 

 be moved in all three ways, and the condyles are not only convex 

 from side to side, and from before backwards, but are set on 

 obliquely, an arrangement which permits all varieties of move- 

 ment. 



The skeleton of the face is also reckoned to include another 

 loose bone, the hyoid (so named from its resemblance to the 

 Greek letter Ypsilon, Y, corresponding to our U), which supports 

 the root of the tongue, and keeps the top of the windpipe open. 

 It possesses two pairs of horn-like projections, and is interesting 

 as representing what is a very elaborate apparatus in such lower 

 forms as fishes. 



In the skeleton of the face there are also properly included 

 the minute bones or ossicles which belong to the organs of 

 hearing, and regarding which more will be said elsewhere. 



3. The Ribs, and 4. The Breast-done. — The ribs and breast- 

 bone (sternum) complete the axial endoskeleton. These, with 

 the thoracic part of the backbone, form a firm framework by 

 which the heart, lungs, and other delicate structures contained in 

 the chest are protected from injury. There are twelve pairs of 

 ribs, jointed on to the backbone dorsally, and, except the two 

 last pairs, connected by pieces of gristle (costal cartilages) with 

 the breast-bone. 



Skeleton of the Limbs. — Turning to the skeleton of the 

 limbs (see p. 196), we find that, both as regards the arm and leg, 

 we can distinguish between {a) a firm bony girdle by which the 

 base of the limb is connected with the trunk, and {b) the hard 

 parts within the free or movable portion of the limb. In the 

 case of the arm we have a shoulder-girdle consisting of two 

 bones, the broad triangular shoulder-blade (scapula), placed 

 dorsally and firmly bound by muscles to the trunk skeleton, and 

 the collar-bone (clavicle), which runs across from scapula to 

 sternum. One corner of the triangular scapula is provided with 

 a shallow cup, the glenoid cavity, into which the rounded end 

 of the upper-arm bone fits so as to form a ball-and-socket joint, 

 which allows of very free movement. It is important to notice 

 a bony projection which overhangs the glenoid cavity, and which, 

 from a supposed resemblance to the beak of a raven (Gk. korax, 



