204 



CHORDATE ANATOMY 



Integumental (dermal or cutaneous) muscles in the form of a panni- 

 culus carnosus group appear suddenly in monotremes and marsupials 

 only to disappear in the higher primates except as rudiments. In the 

 head and neck region, however, the platysma and facial muscles persist 

 in man and apes. In the trunk region, these integumental muscles are 

 outgrowths of the pectoralis minor complex. In the head region, however, 

 they are visceral in origin. 



The most important muscular novelty contributed by mammals is the 

 diaphragm. Its innervation by branches of cervical spinal nerves proves 

 that it is a derivative of cervical myotomes. 



Muscles in Man 



There is no essential difference between the muscles of man and those 

 of other mammals. The presence in man of such useless muscle rudi- 

 ments as the sacro-coccygeal and ear muscles suggests a mammalian 

 derivation. The evolutionary process of subdivision, fusion, migration, 

 and sphtting of muscles reaches its cUmax in primates, forearm and hand 

 being especially noteworthy. 



The human body has nearly four hundred paired or bilaterally sym- 

 metrical muscles, of which forty-seven pairs are visceral and the rest 

 skeletal. In addition to these, four unpaired muscles are recognized. 

 Each part of the body — head, neck, back, abdomen, thorax, diaphragm, 

 shoulder and chest, upper arm, forearm and hand, hip, thigh, lower leg 

 and foot, pelvis — has its intrinsic set of muscles. Space does not permit 

 the description of all these muscles. 



There is no question that in fundamental pattern the muscles of 

 vertebrates and of man are alike. (See Fig. 190.) Comparison of the 

 superficial muscles of man (Figs. 194 and 195) with those of the cat 

 (Fig. 190) reveals a surprising degree of resemblance. On account of 

 their exact homology many of these muscles in the two forms are given 

 identical names. The same is true of many of the deeper muscles. 

 When the muscles of man are compared with those of another primate, the 

 similarity is much greater. There is no reason to doubt that the similarity 

 of the mimetic muscles in man and monkey (Fig. 193) has genetic signifi- 

 cance. Few, if any, muscles in man are without homologues among 

 primates. 



The evolution theory in its appHcation to the human body derives much 

 support from the comparative anatomy of the muscles. The presence in 

 man of useless muscle rudiments such as those of the coccyx and ear 

 mentioned above (page 204) receives its only adequate interpretation 

 in this theory. Pointing in the same direction is the existence in man of 

 inconstant and variable muscles, the homologues of which are functional in 



