60 ANNALS NEW YORK ACADEMY OF SCIENCES 



ORIGIN AND INSERTION" 



The origin and insertion of the muscles must be taken into considera- 

 tion in the determination of their homologies, but as some of the skeletal 

 elements shift and drop out in the different classes, this criterion must 

 be used with care in cases where the animals compared are not closely 

 related. 



The known changes and disappearance of the bones of the skull as 

 we pass from Palaeozoic to modern vertebrates suggest that, if the sup- 

 posed homology of a muscle is based on the origin and insertion, the 

 history of the bone to which it is attached must be known and completely 

 traced; for example, in tracing the changes that have taken place in the 

 remodeling of the cynodont skull, in its evolution into the mammalian 

 type, there is difficulty in following the origin and the insertion of cer- 

 tain muscles, as the shifting of bones is so marked in the jaw region that 

 some muscles have changed their origin and insertion and some have 

 dropped out and been replaced by slips from neighboring muscles. Al- 

 though it seems reasonable to infer that muscles became readapted, yet 

 if there was a mechanical or other reason for the dropping out of a 

 muscle we may assume that a new slip was separated from another muscle 

 that filled the requirements of the new function. Great changes from 

 the primitive reptilian type must have taken place in the line leading 

 to Cynognafhus, Gomphognathus and Sesamodon. The posterior end of 

 the dentary increased in importance and the coronoid process of the 

 dentary gradually overshadowed the posterior end of the mandible as an 

 attachment for muscles, so that muscles formerly attached to the sur- 

 angular and to other posterior bones of the jaw moved forward and 

 acquired an attachment on the upgrowing coronoid process. Finally 

 the reduced muscles of the movable pterygoid of the reptiles must have 

 shifted, disappeared or possibly be left as remnants such as the pterygo- 

 spinosus of the edentates or the pterygo-tympanic that is sometimes found 

 in man. 



FUNCTION 



The jaw muscles are on the whole remarkably stable throughout the 

 vertebrates with minor adaptive changes. The larger muscle masses can 

 be traced through the different classes, but some of the minor slips must 

 be followed closely in their development through a number of forms if 

 the homology is to be certain. The history of the changes of function 

 can be traced from their innervation and their relation to their supports. 

 The fact that they are derived from the visceral muscle system indicates 



