- 1The body-wall of a Teleostean fish is composed on either side of a series of transverse muscles (myomeres) divided from one another by fasciæ of connective-tissue (myocommata).For practical purposes these may be regarded collectively as forming a single lateral muscle.
- 2The lateral muscle is composed of a single layer, which varies considerably in thickness in different parts of the body.
- 3In its primary condition, such as obtains in the caudal region, it is divided into two symmetrical longitudinal moieties, which are separated by a horizontal septum passing from beneath the lateral line to the vertebral column.
- 4The dorsal moiety is marked externally by a series of backwardly directed >-shaped bands. By joining the apices of these >'s an imaginary line is formed dividing the dorsal moiety into a (dorsal) mesio-dorsal portion and a (ventral) latero-dorsal portion. In the mesio-dorsal portion the muscle-fibres run from above anteriorly to below posteriorly; in the latero-dorsal portion they take the opposite inclination.Internally the mesio-dorsal portion is recognised as a series of incomplete hollow pyramids, or cones, whose apices are directed posteriorly; while the latero-dorsal portion is composed of a series of complete hollow pyramids, or cones, whose apices are directed forwards.The ventral moiety is likewise divided into a (dorsal) lateroventral portion and a (ventral) mesio-ventral portion. The fibres of the latter, at first oblique, as they approach the middle line become straight, and resemble a rectus.
- 5The above divisions of the dorsal and ventral moieties suffice for descriptive purposes; but, pending further evidence, they must be regarded as essentially empirical in nature.
- 6Small cylindrical longitudinal muscles are frequently cut off from the mesio-dorsal portion, along the mid-dorsal line in the interspaces of the dorsal fins. These are the supracarinales of Owen.Similar muscles are frequently cut off from the mesio-ventral portion along the mid-ventral line. These are the infracarinales of Owen.
- 7A small muscle, whose fibres are usually distinguished by their red colour, is cut off from the latero-dorsal portion and occupies the slope of the furrow in which the lateral nerve is situated.A corresponding muscle is cut off from the latero-ventral portion immediately below the lateral line.