706 THE MUSCULAR SYSTEM 



tail musculature in the vertebrate group as a whole, it is important that one 

 consider the environment in which the various species live, for the trunk and 

 tail musculature is adapted to the general junctions of moving the animal in 

 its particular habitat. We may recognize three main environmental adaptations: 



( 1 ) natatorial, 



(2) terrestrial, and 



( 3 ) aerial. 



a) Natatorial Adaptations. Animals, adapted to swimming, possess a 

 different arrangement of the musculature of the trunk and tail regions than 

 do terrestrial and aerial forms. A transverse section through the tail of the 

 dogfish, Squalus acanthias, demonstrates that the musculature is arranged 

 around the vertebrae in a definite pattern. A horizontal skeletogenous septum 

 extends outward from either side, dividing the muscles on each side of the 

 vertebra into epaxial and hypaxial groups, and dorsal and ventral septa are 

 present in the middorsal and midventral areas (fig. 326C). 



Viewed laterally, the muscles are divided by transverse membranes, the 

 muscle septa, myosepta, or myocommata (figs. 326D; 327A). The position 

 of the myocomma corresponds to the intermyotomic (intersegmented) area 

 observed in Chapter 15. Each myocomma is attached to the vertebral body 

 (really several vertebral bodies). The myotomes (fig. 326D) lie in the seg- 

 mented position between the myocommata and are attached to the latter. In 

 the tail, both these groups of muscles are attached to the myocommata and 

 the vertebrae, but, farther forward in the trunk, it is the epaxial group which 

 is associated directly with the myocommata and the vertebrae, the hypaxial 

 group being less direct in its contact with the vertebral column. (See fig. 311B.) 

 In figure 327B, the myotomes and myosepta (myocommata) have a Z-shaped 

 appearance because of a secondary modification during development. 



It is evident, therefore, that in the shark, the skeletal muscles of the trunk 

 and tail exist in the form of segments, each segment being divided into an 

 upper epaxial and lower hypaxial component. This arrangement of the muscles 

 and the attachment of the fibers to the myosepta, and thus through the 

 myoseptum to the vertebra, produces a mechanism exceedingly well adapted 

 to the side-to-side movement of the vertebral column so necessary during 

 natation. The conditions present in the sharks are comparable to those of 

 other fishes, and, in all, the epaxial musculature is exceedingly well developed. 



b) Terrestrial Adaptations. In the land-frequenting vertebrates, there 

 is less development of and dependence upon the tail region and the dorsal 

 or epaxial musculature for locomotive purposes. In consequence, the epaxial 

 musculature is segregated on either side of the vertebrae in a dorsal position, 

 while the hypaxial musculature and its derivatives in the bilateral appendages 

 are expanded ventrally. The suppression of epaxial muscle development is 

 carried to an extreme form in the aerial adaptations of the bird. In non- 



