MUSCULAR SYSTEM. 119 



The structure of the cranio-visceral musculature of Teleostei 

 differs considerably from that roughly sketched out above, so that 

 the different groups of muscles must be arranged in an entirely 

 different manner. Thus the following divisions may be distin- 

 guished : (1) Muscles of the jaws ; (2) muscles of the dorsal, 

 and (3) muscles of the ventral ends of the visceral arches. 



Each of these groups may again be subdivided, but further 

 details about their arrangement, which is often very complicated, 

 cannot be given here. 



Amphibia. It is to be expected, a priori, that the muscula- 

 ture of the visceral skeleton should be more highly developed in 

 gill-breathing than in lung-breathing Amphibiaus ; we thus find 

 that in the former, more primitive relations are met with, connecting 

 them with lower forms, while jn the latter a greater modification, 

 or rather reduction, of these muscles takes place. 



Between the two rami of the lower jaw there lies a muscle 

 with transverse fibres (the mylohyoid), supplied by the third 

 division of the trigeminal and the facial ; this represents the last 

 remnants of the constrictor muscle of Fishes. As the elevator of 

 the floor of the mouth, it stands in important relation to respira- 

 tion and deglutition, and is retained throughout the rest of the 

 Vertebrata up to Man (Fig. 98, 99, Mh, Mh l ). 



A continuation of the trunk-musculature (the omo-, sterno-, and 

 genio-hyoid) provided with tendinous intersections, lies above the 

 mylohyoid (Fig. 99, Be 1 , G7i). These muscles, which serve to pull 

 the visceral skeleton forwards and backwards, are supplied by the 

 first and second spinal nerves. 



In contrast to Fishes, there is in Amphibia a definite differen- 

 tiation into muscles of the tongue, that is, into a hyoglossus 

 and a genioglossus, but these also must be considered as having 

 been derived from the anterior end of the ventral muscles of the 

 trunk ; they are present in all Vertebrates, from the Amphibia 

 onwards, and are supplied by the hypoglossal (the first spinal nerve 

 of Amphibians). 



In the Perennibranchiata and in Salamander larvaB the muscles 

 of the hyoid and of the visceral arches may, by analogy 

 with Fishes, be divided into a ventral and a dorsal group : the 

 latter disappears in adult Salamanders and Anura, only the ventral 

 persisting. Their function is to raise and depress the branchial 

 arches, as well as to draw them forwards and backwards. To these 

 may be added constrictors of the pharynx as well as (in gill- 

 breathing animals) levators, depressors, and adductors of the 

 external gill filaments (Figs. 98 and 99). They are innervated 

 by the vagus and glossopharyngeal. 



The jaw-muscles may be divided into a depressor (digastric, 

 or biventer mandibulse, Fig. 98, Dg}, supplied by the facial, and 

 into elevators of the lower jaw fmasseter, temporal, and pterygoid 



