176 COMPARATIVE ANATOMY 



subdermal connective tissue, in which they end and usually also 

 arise : these are known as integumentary muscles, the first traces of 

 which are seen in the Anura. Their relations to the integument 

 have apparently been acquired secondarily, and they are to be 

 looked upon, at any rate in the Amniota, as originating from true 

 skeletal muscles : this is most plainly indicated in Monotremes 

 (Fig. 138), in which there is a close connection between the 

 epidermic exoskeleton and marsupial and mammary apparatus on 

 the one hand, and the integumentary musculature on the other. 1 



Apart from the cutaneous striated muscles of the trunk and 

 limbs, an apparatus composed of smooth muscle-fibres is present 

 in Urodeles, and is more highly developed in Reptiles, in connection 

 with the nostrils, serving as dilators and constrictors. In Anurans 

 these muscles have become reduced, and play only a subsidiary 

 part, the movements of the alinasal cartilages here depending upon 

 those of the lower jaw, which presses upon the movable pre- 

 maxillaB and thus effect the closing of the nostrils : their opening- 

 is due essentially to the elasticity of the parts. The only other 

 integumentary muscles amongst the Anura, apart from a superior 

 labial muscle composed of smooth elements, are certain bands in 

 the regions of the trunk (rutaneus ^crtoris, c. atdominis) and 

 thigh (gracilis minor), and these are only present in the higher 

 forms. 



In the Sauropsida the integumentary muscles play a great part 

 owing to their relations to the scutes, scales, and feathers ; and 

 those of Snakes will now be briefly dealt with as an example. 



Considerable variations in arrangement, form, and insertion of 

 the integumentary muscles occur amongst the Ophidia. These 

 differences depend mainly on the number, form, and arrangement 

 of the ventral and lateral scales or scutes, and on the manner in 

 which they are utilised in locomotion according to the habits of 

 the snake in question. The muscles of the skin are most markedly 

 developed in those snakes which can creep rapidly over the ground 

 or burrow under it, and in which, by erection of the ventral scutes, 

 firm points of contact are formed between the hinder edges of 

 the latter and the ground, so that the body can be pulled or pushed 

 forwards (Fig. 137, A). 



The muscles extending from the ribs to the scutes also aid in 

 progression ; they serve to throw the body into curves and to 

 straighten it, to draw it forwards over the integument, and con- 

 versely to move the integument forwards ventrally and laterally 

 over the body, thus aiding in giving the scutes a firm hold on 

 the ground (Fig. 137, B, c). 



The integumentary musculature reaches its greatest develop- 



1 It is, however, held by some Morphologists that the integumentary muscles 

 of Reptiles and Mammals are derivatives of a superficial part of the lateral 

 muscles of Fishes and Amphibians : in certain Anurans, Lizards, and Snakes, 

 relations between the integument and the rectus and superficial external oblique 

 certainly exist. 



