136 COMPARATIVE ANATOMY 



rule flat, while those of the extremities have usually an elongated, 

 cylindrical, or prismatic form. In some cases, however, they 

 assume the most various shapes : for instance, there may be more 

 than one origin (bicipital, tricipital, or quadricipital forms), the 

 belly may be double (biventral or digastric form), or the muscle 

 may be saw-shaped, or have its fibres arranged in a single or 

 double series like a feather. 



All the muscles are surrounded by fibrous sheaths, or fascim, 

 by means of which they are more or less firmly connected with 

 one another and with the integument and skeleton. Wherever a 

 marked friction occurs, ossifications (sesamoids) may become de- 

 veloped in the course of a muscle or tendon. 



The differentiation of independent muscles may take place — (1) by the 

 separation of the originally single muscle into proximal and distal parts by 

 the formation of an intermediate tendon ; (2) by the splitting of a muscular 

 mass into layers ; (3) by a longitudinal splitting ; or (4) by a fusion of distinct 

 muscles. A muscle may undergo very considerable modification both in form 

 and position by a change of origin and insertion ; and when the action of a 

 muscle becomes unnecessary, it either disappears partly or entirely, or what 

 remains of it contributes to the strengthening of a neighbouring muscle. 



The following important factors must be taken into consider- 

 ation in connection with the muscular system : (1) the homologies 

 of the parts of the skeleton ; (2) the relative positions of the 

 neighbouring soft parts ; and (3) the nerve-supply. 



Most of the muscles bear a close relation to the skeleton from 

 which they take their origin and into which they are inserted. 

 The integumentary musculature, on the other hand, lies en- 

 tirely in the subcutaneous connective tissue, but in Mammals its 

 origin can be traced to the deeper, skeletal muscles : this is 

 most plainly seen in Monotremes. Only slightly developed in the 

 Anamnia, it becomes of great importance in Keptiles and Birds on 

 account of its relations to the scutes, scales, and feathers. It is 

 most highly developed amongst Mammals, where it may extend 

 over the back, head, neck, and flanks as the panniculus carnosus 

 (Echidna, Dasypus, Pinnipedia, Erinaceus, &c.). In Man, only a 

 rudiment of this muscle is found in the shape of the platysma 

 myoides, which extends over the neck and part of the breast and 

 face. 



The action of the integumentary muscles is very different in 

 different Vertebrates. It may (1) serve to roll the body up into a 

 ball (e.g., Hedgehog, Armadillo) ; (2) be connected with a tail 

 adapted for swimming (e.g., Ornithorhynchus) ; (3) serve to erect 

 the integumentary spines [e.g., Echidna) ; or (4) cause local move- 

 ments (" twitching ") of the skin (many Mammals). 



The facial muscles, though present in rudiment in the Anamnia, 

 form a marked feature for the first time in Mammals, arising mainly 

 in connection with the platysma myoides, and gradually extending 



