MUSCULAR SYSTEM OF VEETEBRATA. 493 



distinguished by its deeper colour, may be regarded as belonging to 

 this system. In the Amphibia there are dermal muscles in the 

 head, which act on the nasal orifices, and, in the Anura, in the 

 anal region also. The muscles lying on the external nares are better 

 developed in the Reptilia. The dermal muscles are of great func- 

 tional importance in the Ophidii, as they produce a movement of the 

 scales, which is of use in locomotion. 



In Birds there are large flattened dermal muscles in various 

 parts of the body ; as in the Reptilia (Chelonii) there is a continuous 

 layer of muscles in the neck; other dermal muscles take their origin 

 from the skeleton, such, for example, as the musculi patagii, which 

 pass into, and extend the membrane of the wing. The muscles 

 which serve to move the remiges and rectrices also belong to this 

 category. 



The dermal musculature is more highly developed in the Mam- 

 malia. There is generally a large muscle below the integument of 

 the trunk, which covers the dorsal region of the body, and is con- 

 tinued on to the head and neck ; this is inserted by tendinous 

 pieces into different portions of the integument, while anteriorly it 

 is also inserted into the humerus. It is best developed in Echidna, 

 Dasypus, and in Erinaceus, where it forms the muscle by which the 

 body rolls itself up. In most of the Apes the large dermal muscle 

 is distributed over the same parts as in other Mammals, but it is 

 more distinct in its anterior portion. In the Orang and Chimpanzee 

 this latter is represented by a muscular plate, which occupies the 

 sides of the neck, and is continued on to the face ; in Man it is 

 reduced to the platysma myoides. 



Musculature of the Skeleton. 

 § 372. 



The differentiation of the muscles, which is due to the connec- 

 tion between the muscular system and the skeleton, is very inti- 

 mately connected with that of the skeleton, inasmuch as both parts 

 have always reciprocal relations, owiug to their being both formed 

 for the same function. When, therefore, any portion of the skeleton 

 is increased in size, the muscles that go to it are also increased, and, 

 when another part undergoes atrophy, its muscles are correspond- 

 ingly diminished. So, also, the greater functional independence of 

 the muscles is bound up with a greater differentiation. 



This differentiation gives rise to a number of systems of muscles, 

 each of which can be again divided into subordinate complexes of 

 more or less distinct muscles. The muscles of the trunk, of the 

 cephalic skeleton, and of the appendages, may be distin- 

 guished as systems of this kind. 



The above-mentioned primitive musculature gives rise to the 



