MUSCULAR SYSTEM OF VEKTEBEATA. 497 



of the branchial skeleton. Part of these muscles have their origin 

 on the skull, others belong to the different arches, while others again 

 are arranged, transversely, and act so as to approximate the arches of 

 either side to one another. Muscles pass off from the branchial 

 arches to the branchiostegal rays. They are well developed in the 

 Selachii, but rudimentary in Osseous Fishes, where they appear to 

 be converted, on the hyoid arch, into the muscles of the operculum, 

 and of the dermal rays of the gills. The Amphibia are provided 

 with a similar musculature during their larval stages ; this is partly 

 derived from the muscles of Fishes, and is retained by the Perenni- 

 branchiata throughout their life. When the branchial framework 

 disappears, and the hyoid becomes more independent, part of the 

 branchial musculature is taken up by it. 



As to the muscles of the jaw, it can be shown that an adductor 

 of the two parts of the mandibular arch in the Selachii undergoes a 

 certain amount of differentiation into several parts, and forms the 

 rudiment of the muscles of mastication. When the palato- quadrate, 

 or the bones differentiated in it, are fixed to the cranium, these 

 muscles are inserted into the lower jaw. In the Amphibia and 

 Reptilia an inner portion of this mass of masticatory muscles is 

 differentiated as the pterygoid, and this again may be divided into 

 two (Pt. externus and internus) (Saurii) ; the differentiation of the 

 temporal and masseter muscles is indicated by their arrangement in 

 layers. In both classes the depression of the jaw is effected by a 

 muscle, which forms a short but powerful belly on the hinder edge 

 of the lower jaw. It corresponds to the posterior belly of the 

 digastric of Mammals. The Ophidii are distinguished by an increase 

 in the number of the muscles ; in addition to adductors of the 

 lower jaw, special muscles, which move the quadrate and the various 

 bones of the palatine arcade, may be seen to be largely developed 

 in the Eurystomata. In Birds there are similar muscles, which 

 elevate the pterygoids and the quadrate, and produce the movement 

 of the maxillary apparatus. The temporalis is the largest of the 

 proper mandibular muscles, and the adductor, which is present in 

 the lower divisions, where the two halves of the jaw are movable, 

 is replaced by a muscle which extends transversely between the two 

 halves of the jaw, and has a different function. 



The masticatory muscles of the Mammalia are similar in number, 

 origin, and insertion to the same muscles in Man ; as a rule they 

 ai^e larger, but otherwise they do not differ in any points except 

 those which are due to the form of the surfaces of origin and 

 insertion provided by the proper bones. 



§ 376. 



Of the paired appendages, the fins of Fishes possess a number 

 of muscles on the girdle, as well as on the free portion, but it has 

 not been possible to compare these muscles with those of other 



2 K 



