322 



ELEMENTARY PHYSIOLOGY 



its own axis, resting on another bone. A remarkable 

 example of the former ari'angement is afforded by the 

 atlas and axis, or two uppermost vertebrjB of the neck 

 (Fig. 100). The axis possesses a vertical peg, the so-called 

 odontoid process (h), and at the base of the peg are two, 



Bi- -1 



flo. 90.- lonoitudinal and vertical .section through the 

 Elbow-joint. 



ff, humerus ; Ul. ulna ; Tr. the triceps muscle, which extends the arm, 

 Bi, the biceps muscle, which flexes it. 



obliquely placed, articular surfaces (a). The atlas is a 

 ring-like bone, with a massive thickening on each side. 

 The inner side of the front of the ring plays round the 

 neck of the odontoid peg, and the under surfaces of the 

 lateral masses glide over the articular faces on each side 

 of the base of the peg. A strong ligament passes between 



