47 



and then again leaves that spine and runs obliquely forwards. Thus each transverse 

 membrane is attached by its inner edge to two adjacent vertebral spines at different 

 parts of its length, and along the rest of its length is continuous with the median fibrous 

 membrane which extends between the spines and the interspinous bones. 



The anterior segments of the dorsal lateral muscle are much elongated and bent 

 forwards, so that their separating membranes are attached to the dorsal surface of the 

 skull, the anterior interspinous bones and the membrane connecting the latter. 



The anterior segments of the ventral muscle covering the visceral region are straighter 

 than the posterior, and the inner edges of the dividing membranes are not attached to 

 any firm skeletal structure, but simply to the fibrous membrane which encloses the body- 

 cavity, the peritoneal membrane. The ventral part of the ventral lateral muscle on 

 each side is also separated from the median skeletal partition by the posterior prolonga- 

 tion of the body-cavity. 



The posterior muscle-segments, in correspondence with the greater backward slope 

 of the vertebral spines, are folded to a much greater extent than the anterior. The 

 terminal segments form a system of muscles which are attached to the bases of the 

 caudal fin-rays. 



It follows from the description that in each lateral muscle there are as many muscle 

 segments as there are vertebrae, and that each dividing membrane corresponds to a 

 vertebra Thus the muscle segments themselves correspond to the junctions between the 

 vertebral centra, an arrangement which allows the vertebral column to be bent in all 

 directions by the action of the lateral muscles. It is clear that, although it is difficult to 

 understand in detail the effect of the complicated attachments above described in the 

 contraction of the muscles, the general effect of the contraction of the lateral muscles of 

 one side is to bend the body powerfully towards that side, and it is by this alternate 

 bending of the body first to one side then to the other that the sole swims through the 

 water when it rises from the bottom, or swims along the bottom. The lateral muscles 

 form the bulk of the edible portion of the fish. The lateral muscles of the left or lower 

 side do not differ in any important respect from those of the right. 



The other muscles of the sole are the muscles of the fins and the muscles of the 

 ventral region of the head, and the eye muscles. 



The muscles of the longitudinal fins are well developed and important. The caudal 

 fin-rays are, as has been mentioned, moved by muscles which represent the terminal 

 muscle segments of the lateral muscles. The dorsal and anal fin-rays are moved by two 

 distinct systems of muscles. One system serves to erect the rays and to depress them 

 by causing them to slope backwards till they are almost parallel to the edge of the 

 body. The other system moves the rays away from the median plane, either to the right 

 side or to the left (upwards or downwards in the natural position of the sole). The 

 muscles of the former system may be called the elevators and depressors of the fin-rays, 

 those of the other the right and left abductors. The abductors of the fin-rays are 

 superficial, lying immediately beneath the skin ; there is a single muscle on each side 



