CHAPTEE Y. 



MYOLOGY. 



In the lowest vertebrate, Branchiostoma, the whole of the 

 muscular mass is arranged in a longitudinal band running 

 along each side of the body; it is vertically divided into 

 a number of flakes or segments {myocomvias) by aponeu- 

 rotic septa, which serve as the surfaces of insertion to the 

 muscular fibres. But this muscular band has no connection 

 with the notochord except in its foremost portion, where 

 some relation has been formed to the visceral skeleton. A 

 very thin muscular layer covers the abdomen. 



Also in the Cyclostomes the greatest portion of the mus- 

 cular system is without direct relation to the skeleton, and, 

 again, it is only on the skull and visceral skeleton where 

 distinct muscles have been differentiated for special functions. 



To the development of the skeleton in the more highly 

 organised fishes corresponds a similar development of the 

 muscles ; and the maxillary and branchial apparatus, the 

 pectoral and ventral fins, the vertical fins, and especially the 

 caudal, possess a separate system of muscles. But the most 

 noteworthy is the muscle covering the sides of the trunk 

 and tail (already noticed in Branchiostoma), which Cuvier 

 described as the "great lateral muscle," and which, in the 

 higher fishes, is a compound of many smaller segments, cor- 

 responding in number with the vertebrae. Each lateral 

 muscle is divided by a median longitudinal groove into a 

 dorsal and ventral half; the depression in its middle is 



