176 STRUCTURE OF THE VERTEBRATES 



group. Reference to Amphioxus will make the relationships 

 clear. The embryos of all vertebrates are similar in this general 

 plan, although the adults vary widely in the degree of speciali- 

 zation from this primitive condition. 



The striated muscles are classified both by their origin and 

 their position in the body. In the more primitive vertebrates the 

 topographic arrangement of the muscles will usually indicate 

 their origin and homology, but in the higher groups the position 

 of the muscles has become more highly specialized. Therefore 

 the anatomist must depend upon embryology and comparative 

 anatomy to determine the correct relationships of the individual 

 groups. Four major groups of skeletal muscles are usually 

 recognized. 



1. Axial muscles are the original metameric muscles which 

 arise from the epimere and lie along the axis of the body. They 

 are primitively metameric, although the metamerism is largely 

 lost or obscured in the higher classes. 



2. Branchiomeric muscles arise from the hypomere in con- 

 nection with the branchial, or gill, cartilages. After the disap- 

 pearance of the gills and the modification of the cartilages, these 

 muscles are retained in the jaw and throat. 



3. Appendicular muscles arise from the primitive axial group 

 and grow distall}^ on to the appendages. They are so highly 

 modified that they are classified as a separate major group of 

 muscles. Embryologically they can be traced to the epimere and 

 are classified on the basis of their topographic arrangement. 



4. Integumentary muscles arise from all three of the above 

 groups. They become attached to the skin of the animal and 

 form a distinct functional group. 



B. NOMEXCLATURE AND FUNCTION 



There is no system of the body in which a knowledge of 

 homologies between individual structures is as incomplete as in 

 the muscles. The major topographic divisions can easily be 

 followed from class to class, but homologies between individual 

 muscles are not always certain. Homologies between reptiles and 

 mammals are particularly difficult. This is caused by the fact 

 that each muscle is not always clearly defined, for in the course 



