lU COMPARATIVE ANATOMY 



ing them with lower forms, while in the latter a greater modification, 

 or rather reduction, of these muscles takes place. 



Between the two rami of the lower jaw is situated a muscle with 

 transverse fibres (the mylohyoid), supplied by the third division of 

 the trigeminal and the facial nerve ; this represents the last rem- 

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

 floor of the mouth, it stands in important relation to respiration and 

 deglutition, and is retained throughout the rest of the Vertebrata 

 up to Man (Figs. 116, 117). 



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

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

 mylohyoid (Fig. 117). 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 Amphibians 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 larvs the muscles 

 of the hyoid and of the visceral arches may, as in Fishes, be 

 divided into a ventral and a dorsal group ; the latter disappears in 

 adult Salamanders and Anurans, only the ventral persisting. Their 

 function is to raise and depress the branchial arches, as weU as to 

 draw them forwards and backwards. To these may be added 

 constrictors of the pharynx, as well as (in branchiate forms) 

 levators, depressors, and adductors of the external gill filaments 

 (Figs. 116 and 117). They are innervated by the vagus and 

 glossopharyngeal. 



The jaw-muscles include a depressor {digastric, or Mventer 

 mandibulce, Fig. 116), supplied by the facial nerve, and 

 elevators of' the lower jaw {masseier, temporal, and pterygoid 

 muscles), supplied by the third division of the trigeminal. All 

 these muscles, which may be derived from the adductor of the 

 mandible of Elasmobranchs and Ganoids, arise from the auditory 

 region of the skull. 



Amniota. — With the simplification of the visceral skeleton in 

 Amniota there is a considerable reduction of the musculature 

 belonging to it. All muscles connected with branchial respiration 

 are of course wanting, and the ventral trunk-muscles, as mentioned 

 above, are always interrupted in their forward extension by the 

 sternum and pectoral arch. At the same time, the muscles along 

 the neck and on the floor of the mouth met with in Amphibia are 

 present here also ; they are, a mylo-, sterno-, omo-, and genio- 

 hyoid, as well as a hyoglossus and genioglossus. To these may 



