CHAPTER V.
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 (myocommas) by aponeurotic 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 muscular 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, corresponding in number with the vertebræ. Each lateral muscle is divided by a median longitudinal groove into a dorsal and ventral half; the depression in its middle is
