246 



HISTORY OF THE HUMAN BODY 



greatly simplified form, and the few muscles that persist enter 

 into the service of aerial respiration and regulate the opening 

 and closing of the pharyngeal cavity and the larynx. Among 

 them appear two well-defined series of muscles, the one dorsal 

 and the other ventral to the visceral arches, that act respectively 

 as levators and depressors of those parts. Their condition in 

 urodeles, together with a diagram representing an hypothetical 



FIG. 66. Diagrams of primitive visceral muscles. 



(A) Typical form, hypothetical. (B) Condition based upon that of the urodele 

 Siren, with a few details supplied from Necturus. 



I-VIIA, levators of the arches; I-VIIv, depressors of the arches; m, mandibular 

 arch; h, hyoid arch; bi to i> 7 , branchial arches; t, trigeminus; fa, facialis; gl, glosso- 

 pharyngeus; v, to v t vagus (pneumogastric) elements; x, temporalis; y, masseter; s, 

 digastricus; la 1-4, levatores arcuum; dl, dorso-laryngis and dorso-trachealis; a, inter- 

 mandibularis anterior; c, intermandibularis posterior; d, hyo-pharyngeus, anterior por- 

 tion; e, hyo-pharyngeus, posterior portion; /, laryngei. 



ancestor from which may have been derived, are given 

 in Fig. 66. 



In the diagram A the seven visceral arches, including the 

 mandible, are given in order, representing as many somites, 

 with their motor nerve supply. For the first or mandibular 

 somite this latter is the mandibular branch of the trigeminus; 

 for the second or hyoid, the facialis; for the third, the glosso- 



