THE MUSCULAR SYSTEM 249 



layer are derived the pharyngeal constrictors, and it is probable 

 that the intrinsic muscular fibers which make up the mass of 

 the tongue and known as the lingualis, come from the -same 

 source. 



Superficial to the muscular systems already described, and 

 lying directly beneath the integument there are found in many 

 vertebrates muscular elements, usually in the form of sheets 

 or layers, and connected with the integument, which thus ac- 

 quires locally some power of movement. These muscles form 

 what may be conveniently termed the integumental system, 

 although there are included here contributions from several 

 wholly unrelated systems, independently developed in the dif- 

 ferent groups of animals to subserve special functions and 

 therefore restricted in their occurrence. These integumental 

 muscular elements have arisen from whatever preexisting 

 muscles happen to be adjacent to the location where such a part 

 is needed, and thus they may be in their origin either axial, 

 visceral or appendicular, or may represent a combination of 

 these systems. They usually possess at one end a firm attach- 

 ment to some skeletal part or at least to skeletal muscles, while 

 at the other end, or perhaps along an extended surface, they 

 adhere to the inner side of the integument, thus furnishing 

 the skin area involved with the degree of motion required. 



In both birds and mammals certain shoulder muscles furnish 

 an important contribution to the integumental system, but, 

 as would be expected, the two cases are totally independent 

 of one another. In birds the integumental area involved is 

 the patagium or web, extending across the angles of the 

 axilla and elbow and increasing the resisting surface of the 

 wing. This is regulated by a series of patagial muscles, strictly 

 integumental in their relations, but derived from the various 

 muscles of shoulder and arm ; of these the most important 

 are M. propatagialis, derived from the anterior portion of the 

 pectoralis, and an associated slip from the biceps. 



In mammals an extensive layer, derived from latissimus and 

 pectoralis, spreads over the side of the body, and in some 

 cases the two extend to the mid-dorsal and mid-ventral lines, 



