MUSCULAR SYSTEM 189 



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

 biventer mandibulcc, which here has only a single belly, Fig. 140), 

 supplied by the facial nerve, and into several elevators of the lower 

 jaw (masseter, temporal, and pterygoid muscles), supplied by the 

 third division of the trigeminal. The last-mentioned muscles may 

 be derived from the adductor of the mandible of Elasmobranchs, 

 and the biventer from the portion of the superficial constrictor of 

 Fishes which passes to the lower jaw : it arises from the same 

 matrix as the platysma, and serves to open the mouth. 



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 the mylo-, sterno-, omo-, and genio- 

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

 be also added a sternothyroid, and a thyrohyoid continued for- 

 wards from it. 



The stylohyoid, styloglossus, and stylopharyngeus of Mam- 

 mals, arising from the styloid process and stylohyoid ligament and 

 undergoing numerous variations, are peculiar to Mammals. They 

 are supplied partly by the facial nerve, partly by the glossopharyn- 

 geal, and act as retractors of the tongue and levators of the pharynx 

 and hyoid. 1 



The muscles of the jaws resemble those of Amphibia, although, 

 especially in the case of the pterygoids, they are much more sharply 

 differentiated into superficial and deep or external and internal 

 portions, and may become subdivided secondarily (e.g. in the region 

 of the temporal muscle) : they are throughout more strongly 

 developed. 2 



1 For the tensor tympani and stapedius muscles, cf. under Auditory Organ. 

 The latter muscle, together with the stylohyoid, is possibly derived from the dorsal 

 portion of the deep constrictor layer of Fishes which passes to the hyoid, but 

 more probably corresponds to the ventral portion of this muscle. 



- An anterior belly of the biventer appears in Mammals in consequence of a 

 shifting of the superficial layer of the mylohyoid, the fibres of which are originally 

 transverse. Its connection with the tendon of the posterior belly is therefore 

 secondary, as are also the relations of the mylohyoid to the hyoid bone. 



