THE MUSCULAR SYSTEM 247 



pharyngeus; and for the remaining four, a like number of 

 branches from the vagus, which is a complex of several original 

 elements. The dorsal muscles attached to the arches are 

 Icvators, the ventral depressors. In the second figure, B, is 

 shown the actual condition in urodeles, the derivation of which 

 from the first is obvious. 



Beginning with the levator series, the first becomes the equiv- 

 alent of the adductor mandibulce of fishes, here differentiated 

 into tcmporalis and masseter, the muscles of mastication. The 

 second, having its primary connection with the hyoid arch, be- 

 comes also attached to the mandible, but in such a way that it 

 opens it, thus acting as the antagonist to the first. This mus- 

 cle is usually referred to as the " digastric," a name taken from 

 human anatomy, but it is probably homologous with the pos- 

 terior belly alone of the mammalian muscle of the same name. 



The next four muscles are those associated with the first four 

 gill-arches, and function in the lower urodeles and in the larvae 

 of the more specialized ones as the levatores arcuum; the next 

 and last belongs plainly in the same series, but as its arch has 

 become specialized as the primary laryngeal cartilage [Chapter 

 V], it extends ventrally to meet it. On account of this rela- 

 tionship it has received the name of dorso-laryngeus. 



The ventral series consists of flat sheets, arising from the 

 mid-ventral line, where they meet in pairs. Of these, the first 

 two, the intermandibulares, anterior and posterior, form the 

 muscular floor of the mouth and are attached respectively to 

 the mandible and the hyoid. Of the next two, those associated 

 with the two first gill-arches, there is no trace, and the next, 

 the fifth, is present only in Necturus and its ally, Proteus, the 

 lowest of the urodeles. The sixth, under the often inappro- 

 priate name of hyo-laryngeus, is generally present, and the 

 seventh, stretching between the two lateral laryngeal cartilages, 

 becomes a set of true laryngeal muscles, the dorsal and ventral 

 laryngei (laryngcus dorsalis and laryngeus ventralis). 



Above this stage the further phylogenetic history of the 

 visceral muscles has been followed only in part, and the con- 

 clusions drawn are those which are the most obvious. 



