248 HISTORY OF THE HUMAN BODY 



The two masticatory muscles, temporalis and masseter, occur 

 in all higher forms and are homologous throughout, save that 

 two farther slips, the pterygoideus externus and interims, be- 

 come differentiated from them, probably from the original 

 masseter. In mammals a small slip from the pterygoideus 

 internus, becomes the tensor tympani of the middle ear. The 

 second levator becomes associated with a muscular slip from 

 the first depressor, intermandibularis anterior, and forms the 

 digastricus of mammals, the two elements being united by a 

 tendon. The diploneuric character of this muscle, that is, the 

 innervation of the anterior belly from the trigeminus and that 

 of the posterior from the faciaKs, receives thus an explanation. 

 A portion of the posterior belly, that is, of the second levator, 

 becomes separated from it in reptiles, and follows the stapes 

 into the middle ear, whence it becomes the stapedius muscle, 

 innerved by a special branch of the facialis. The ventral mus- 

 cles of these same first two segments are perpetuated, the first 

 in part as the anterior belly of the digastricus just mentioned 

 and in part as the mylo-hyoideus; the second as the stylo- 

 hyoideus. Beyond this, however, the history is not clear. In 

 the previous chapter it was shown that the various gill-arches, 

 beginning posteriorily, become associated with the original 

 pair of laryngeal cartilages to form the complicated larynx of 

 higher forms, but whether the muscular elements primarily 

 \ associated with them assist in the formation of the musculature 

 ^ of the final organ, or whether this musculature is derived 

 entirely from the muscles primarily belonging to the seventh 

 arch, that is, dorso-laryngeus and the laryngei, cannot yet be 

 definitely stated. 



The musculature of the tongue, especially its extrinsic 

 muscles, such as hyo-glossus, genio-glossus, stylo- glossus, etc., 

 is probably derived from the visceral muscles, but here another 

 element is introduced, and that is the muscular layer of the 

 anterior end of the alimentary canal, which, although of 

 mesenchymatous origin, and primarily composed of unstriated 

 cells, involuntary in their action, are yet capable of acquiring 

 striae and of becoming at least semi-voluntary. From this 



