ADAMS, PHYLOGENY OF THE JAW MUSCLES 141 



exceptions to this grouping are the mimetic muscles of the face in mam- 

 mals, which represent the muscles from the second constrictor, innervated 

 by the facialis, that have extended over the face and taken their nerves 

 with them. 



If we assume that the jaw and hyoid elements represent the first two 

 visceral arches that have changed their function in some pregnathostome 

 stage, it is also fair to assume that the muscles went with them, and that 

 the jaw muscles represent the much divided and specialized constrictors 

 and adductors of primitive branchial arches. Vetter's analysis of the 

 muscles of the visceral arches appears to hold good at the present day. 

 He held that the adductor mass of the jaws of fishes was homologous with 

 the "adductores arcuum visceralium," which were small muscles on the 

 inner side of the branchial arches, "mittlere Beuger der Bogen," lying 

 between the dorsal constrictors above and the ventral constrictors below. 

 As these branchial arches changed into jaws and the development of the 

 hyomandibular, quadrate and opercular elements proceeded, the con- 

 strictor and adductor muscles, which were already in position, would re- 

 quire but little change to take on new functions as jaw and opercular 

 muscles. Accordingly, the constrictors of the pre-gnathostomes may be 

 regarded as the primitive head muscles. In the elasmobranchs the con- 

 strictor became divided into different sections, so that there were dorsal, 

 median and ventral sections, which were either further differentiated to 

 be used as needed or held in reserve for future changes, as in the case of 

 the digastric muscle. This is represented in the elasmobranchs (Ruge, 

 1897) by the undifferentiated second constrictor, which retains its con- 

 strictor-like form until it becomes specialized in the amphibians into a 

 definite muscle. 



Throughout the gnathostomes we find muscle masses that may be ho- 

 mologized, but the homology of the separate slips is often questionable. 

 In this paper the attempt is made to work out the homology of the main 

 muscle masses ("Mother" masses) first and then, if possible, to point out 

 the homologies of the separate slips of each mass, especially in the mem- 

 bers of the same class and where possible between classes. To homologize 

 the separate slips through the different classes means that one must know 

 the ancestry and be able to demonstrate the movements and changes in the 

 osteology, so that in default of such knowledge the result is often ques- 

 tionable. 



The muscles of the piscine head may be divided as follows (the related 

 muscles are placed together) : 



