258 MR. ST. GEORGE MIVART ON [Apr. 22, 
between the dorsal furrow and the lateral linear division) extends 
from the upper surface of the skull to the distal end of the dorsum 
of the tail (figs. 1 & 8, #. 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 spine of higher animals, but not differentiated into distinct 
muscles. Its fibres are all antero-posteriorly directed. 
Each ventral portion of the lateral muscular mass (7. e. the part 
between the lateral linear division and the middle of the ventral sur- 
face 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, inter- 
rupted, 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, Hx. O). This is an 
elongated sheet of muscular fibres (with the usual direction down- 
wards, backwards, and mesiad) extending from the lateral longi- 
tudinal furrow to the outer margin of the rectus. The muscle ex- 
tends 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 
the fascia investing the root of the thigh, also with the ilium and the 
pubis. It fuses so gradually with the rectus that I have not suec- 
ceeded in defining the line of demarcation between the two. The ex- 
ternal 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. 
At 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, S¢. H). 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 
