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Show 258 MR. ST. GEORGE MIVART ON [Apr. 22, between the dorsal furrow and the lateral linear division) extends from the upper surface of tbe skull to the distal end of the dorsum of the tail (tigs. 1 & 8, E. S), investing the transverse processes, neural arches, and spines of the vertebrae, and also the ribs, but it has no direct connexion with either the pectoral or pelvic girdle. It forms a continuous and very thick fleshy mass, answering to the erector spince of higher animals, but not differentiated into distinct muscles. Its fibres are all antero-posteriorly directed. Each ventral portion of the lateral muscular mass (i. e. the part between the lateral linear division and the middle of the ventral surface of the body and tail) presents a larger surface than does the dorsal portion of the lateral muscular mass. It extends from the basihyal to the distal end of the ventral surface of the tail, interrupted, however, by the pelvis. As in higher vertebrata, the abdominal portion of this mass is distinguished into muscular layers with differently directed fibres ; in the caudal region, however, as might be expected, this distinction does not obtain. MUSCLES OF THE TRUNK. The external oblique (figs. 2, 3, 5, & 7, Ex. O). This is an elongated sheet of muscular fibres (with the usual direction downwards, backwards, and mesiad) extending from the lateral longitudinal furrow to the outer margin of the rectus. The muscle extends forwards as far as the scapula (passing beneath the latissimus dorsi), its anterior extremity being overlapped by the minute and oppositely directed serratus magnus. Posteriorly it is inserted into tbe fascia investing tbe root of the thigh, also with the ilium and the pubis. It fuses so gradually with the rectus that I have not succeeded in defining the line of demarcation between the two. The external oblique appears to be continued on in the caudal region as the most external portion of its infero-lateral muscular mass. This caudal continuation, however, takes fresh origin from the hinder side of the ilium. The internal oblique (fig. 5, In. O) is the largest muscle of the body, and extends antero-posteriorly from the basihyal backwards to the end of the tail (for the pelvis seems only partially to interrupt it), and inwards from the lateral longitudinal furrow. The fibres are but very slightly oblique, very many are altogether antero-posterior in direction, and the fasciculi are very large and coarse. The muscle is thickest anteriorly and in the tail. In the pelvic region many fibres are inserted into the ilium and into the rib which articulates with the ilium, but other fibres appear to continue on altogether beneath these bones without being interrupted by them. A\t its anterior end this muscle becomes indistinguishably fused with what may be a deeper part of the rectus, and which has here been called the sterno-hyoid (fig. 2, 87. II). This is inserted into the basihyal and into the adjacent root of the first branchial arch. Transversalis. A very delicate lamella of transversely directed muscular fibres represents this muscle. It extends in a continuous |
