STRUCTURE OF THE VERTEBRATES 



183 



short vertebral muscles which attach to the spines of the verte- 

 brae. Lateral to these in the anterior half of the body are the long 

 muscles of the back, the longissimiis dorsi, composed of a num- 

 ber of slips. In the posterior half of the back, in addition to the 

 short spinal muscles, are two major groups: (1) the midtifidus 

 and (2) the sacro-spinalis, which pass from the sacrum and 

 pelvic girdle to the transverse processes of the vertebrae, and 

 give leverage to the back. This is, usually, the strongest muscle 

 group of the body. 



Fig. 103. Axial Muscles of the Human Embryo. The primitive myotomes 

 bend, elongate and fuse to form the long axial muscles of the mammal. 



The caudal muscles are continuations of the axial muscles. 

 In the reptiles the body decreases in size toward the tip of the 

 tail without any definite break in continuit}' at the pelvis. In 

 the mammals the tail is less prominent, and does not serve as a 

 locomotor or balancing organ as in the reptiles; and as the tail 

 degenerates these muscles become less and less prominent. A 

 functional exception exists in the South American monkeys, and 

 other mammals with prehensile tails, for in these the caudal 

 muscles are well developed to the tip; but in these animals the 

 tail is approximately the same size throughout its length, and is 



