The Frog 



75 



called .zygapophyses ( ). The little bones themselves 



are held together by ligaments and move on one another by means of the 

 centrum and zygapophyses. This permits a firm axial support, while 

 also allowing for the bending of the body. By having all the vertebrae, 

 one immediately above the other, the neural opening is continuous, so 

 that the spinal cord not only lies free, but the vertebrae themselves are 

 thus prevented from bending sufficiently to damage the cord. 



The surfaces of the centra unite by a ball-and-socket joint. Each of 



the first seven vertebrae possesses a 

 ball on the posterior and a socket 

 on the anterior surface. The eighth, 

 however, is concave on both sur- 

 faces and the ninth is convex on 

 both. It is important to know the 

 difference in action which this en- 

 tails. Although all nine vertebrae 

 are much alike, they can easily be 

 distinguished from one another. The 

 first possesses no transverse process, 

 while the centrum of the ninth has 

 two convex posterior surfaces and 

 very large transverse processes. It 

 is from this last vertebra that the 

 urostyle, the long slender bone, ex- 

 tends backward to the end of the 

 body. 



The urostyle is supposed to rep- 

 resent the tail found in allied ani- 

 mals, such as the salamanders. The 

 spinal cord actually extends into 

 the urostyle, but passes out almost 

 immediately through two small 

 openings on either side, as two 

 rather tiny filaments. 



There are no ribs in the frog, 

 and the transverse processes end 

 rather abruptly a very short distance from the centrum. 



Fig. 21. 

 Pectoral Girdle, Arm, and Hand, of Frog. 



A. The shoulder girdle of the frog; the 

 scapula and suprascapula are turned outwards. 

 ep., episternum; os, omosternum; ep.c, 

 epicoracoids; mes., mesosternum; xi., xiphister- 

 num; s.sc, suprascapula; sc, scapula; gl., 

 glenoid cavity; cor., coracoid; cl., clavicle. 



B. Forearm and hand of right side, as seen 

 from above; ru., radio-ulna; / — V., the five 

 digits; r., radiale; im., intermedium; u., ulnare; 

 a., first distal carpal bone; b., second distal; 

 c, third distal. 



C. Radio-ulna of right side: o., olecranon; 

 r., radius; u., ulna. 



D. Humerus: h., head; sh., shaft; ar., dis- 

 tal articular knob; t., trochlea. (From Bourne, 

 after Ecker.) 



THE APPENDICULAR SKELETON 



The shoulder, or pectoral girdle, ( ) (Fig. 21) 



serves as an attachment for the muscles which move the fore limbs, and 

 also as a protection for the organs in the anterior portion of the trunk. 



The girdle itself surrounds the body just back of the head, consist- 

 ing of a paired scapula ( ), the dorsal part of which 



