INTEODUCTION. 



31 



longitudinally, transversely, and obliquely, the longitudinal 

 and transverse muscles crossing each other at nearly right 

 angles, the oblique ones tending to cross at various angles, as 

 in the letter X. The crossing is seen to most advantage in 

 the deep 'muscles. 



In order to understand the twisting which occurs to a 

 greater or less extent in the bodies and extremities (when 

 present) of all vertebrated animals, it is necessary to reduce the 

 bony and muscular systems to their simplest expression. If 

 motion is desired in a dorsal, ventral, or lateral direction only, a 

 dorsal and ventral or a right and left lateral set of longitudinal 

 muscles acting upon straight bones articulated by an ordinary 

 ball-and-socket joint will suffice. In this case the dorsal, 

 ventral, and right and left lateral muscles form muscular cycles ; 

 contraction or shortening on the one aspect of the cycle being 

 accompanied by relaxation or elongation on the other, the 

 bones and joints forming as it were the diameters of the 

 cycles, and oscillating in a backward, forward, or lateral 

 direction in proportion to the degree and direction of the 

 muscular movements. Here the motion is confined to two 

 planes intersecting each other at right angles. When, how- 

 ever, the muscular system becomes more highly differentiated, 

 both as regards the number of the muscles employed, and the 

 variety of the directions pursued by them, the bones and 

 joints also become more complicated. Under these circum- 

 stances, the bones, as a rule, are twisted upon themselves, 

 and their articular surfaces present various degrees of spirality 

 to meet the requirements of the muscular system. Between the 

 straight longitudinal muscles, therefore, arranged in dorsal and 

 ventral, and right and left lateral sets, and those which run in a 

 more or less transverse direction, and between the simple joint 

 whose motion is confined to one plane and the ball-and-socket 

 joints whose movements are universal, every degree of obli- 

 quity is found in the direction of the muscles, and every, pos- 

 sible modification in the disposition of the articular surfaces. 

 In the fish the muscles are for the most part arranged in 

 dorsal, ventral, and lateral sets, which run longitudinally ; and, 

 as a result, the movements of the trunk, particularly towards 

 the tail, are from side to side and sinuous. As, however. 



