X. Comparison of the Muscles of the Chelonian and Huh an 

 Shoulder-girdles.* By Henry Shaler Williams. 



Presented January, 1872. 



The object of the following paper is to show the importance of tlie 

 positions and rehxtions to each other, and to the axes of the bones, of 

 the areas of origin and insertion of mnscles. 



While comparing the muscles of the Chelonians with those of man, 

 the writer observed that Avhile the bones were found to differ much in 

 slia])e and proportions, and the size, form and number of the muscular 

 bundles, and their relations to each other, were often found to differ, 

 the relations of the areas of origin to each other were found to be 

 remarkably constant. Hence in dissecting out tlie muscles of the Che- 

 lonians from the Ijody outward, or, in other words, tracing the mus- 

 cles from the origin of motion to the part moved, it was observed 

 that the areas of origin numbered 1, 2, 3, ifec, on each bone, as they 

 were exposed, belonged to muscles which were, in final action, very sim- 

 ilar in all, however much they might differ in their size and strength 

 and shape, and even insertion, in the different genera. Then came 

 the assumption that the fundamental reason why muscles in different 

 vertebrate animals should receive the same name is that they per- 

 foi'm the same functions, or that their final action is the same ; and in 

 conclusion, we reach the rule that the areas of origin {oi-, in general, of 

 the attachment) of muscles furnish the most exact means for deter- 

 mining the homologies existing in the muscular systems of difierent 

 forms of animals. 



To apply and illustrate this rule, we take the unique sliouldcr-gir- 

 dle of the Chelonians and compare it with that of man. 



The shoulder-girdle of man is composed, on each side, of a scap- 

 ula (PI. 12, figs. 1 and 2), which alone supports the fore limb, and a 

 clavicle which articulates with a process of the scapula and connects 

 it with the sternum. From, the scapula there arises a process from 

 tlie median line of its posterior surface, called the spine (PI. 12, fig. 1, 

 s.), which extends outward into a [irocess called the acromion process 

 (PI. 12, fig. 1, a.). 



From the superior border, next the glenoid fossa, another ])rocess 



* Abstract of a portion of a Thesis presented to the Sheffield Scientific School, when 

 a candidate for the degreo of Ph.It., July, 1871. 



