188 COMPARATIVE ANATOMY 



partially interrupted by the sternum and pectoral arch, and is 

 continuous with the sternohyoid. These different conditions of 

 the muscles result from the varied adaptations of the visceral 

 skeleton and respiratory organs. 1 



Amphibians. It is to be expected, a priori, that the muscu- 

 lature of the visceral skeleton should be more highly developed in 

 branchiate than in air-breathing Amphibians ; in the former, 

 more primitive relations are met with, while in the latter a greater 

 modification, or rather reduction, of these muscles takes place. 



The muscles of the hyoid and branchial arches may be divided 

 into three groups a dorsal (levatores arcuum), a middle (muscles 

 of the external gills and the external ceratohyoid), and a ventral 

 (internal ceratohyoid, subarcuales, and interbranchialis 3 or 4). The 

 nerve-supply of the dorsal group is strictly branchiomeric ; in the 

 middle, and still more in the ventral group, this condition is not 

 retained. 



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

 with transverse fibres (the myloJiyoid or intermandibular muscle), 

 supplied by the third division of the trigeminal and the facial 

 nerve ; this represents the last remnants of the ventral superficial 

 constrictor muscle of Fishes. As 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. 140, 141). 



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

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

 mylohyoid (Fig. 141). 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 differ- 

 entiation into muscles of the tongue, that is, into a hyoglossus and 

 a genioglossus ; these also must be considered as originating 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 that is, the first (or second, Anura) 

 spinal nerve. 



In the Perennibranchiata and in Salamander larvae 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 Anurans, 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, 

 in branchiate forms, levators and adductors of the external gills 

 (Figs. 140 and 141). They are innervated by the vagus and 

 glossopharyngeal. 



1 The visceral muscles of Polypterus are of especial interest, as they present 

 an intermediate condition between those of Elasmobranchs and Urodeles. 



