154 DR. J. MURIE ON THE FORM AND STRUCTURE OF THE MANATEE. 
genera of the group, Halicore and Rhytina, examined in the flesh. From the apparent 
correlation of rib-structure in the ancient Halitherium, doubtless it also was similarly 
constructed. 
_ This characteristic formation of the thoracico-abdominal parts, alluded to by many, 
has hitherto not been figured, at least its position to the viscera in situ. The 
desideratum I have to some extent supplied in the several sketches Pl. XXVI. figs. 44, 
37, 49, and 50, and also partially in Pl. XXIV. fig. 30. 
These demonstrate, as regards the diaphragm, that it extends in a retrogradent 
tolerably horizontal plane from the first to the last rib, and forms a nearly mesial 
septum separating the lungs within an upper, and the rest of the viscera within a 
lower compartment. Altogether it is very tendinous—though Daubenton’ says “je n'y 
ai point apercu de centre nerveua,” meaning, I apprehend, that there is no ordinary 
free central tendon; but instead, as fig. 44, Pl. XX VI. exhibits, the elongated middle 
tendinous raphe is fastened to the vertebral bodies, and to the keels from the fourth to 
the last dorsal vertebra. 
The fleshy portion of the diaphragm forms a narrow band on each costal margin for 
its entire length. In the larger (female) specimen the muscular breadth measured one 
inch anteriorly, by degrees increasing to two or more towards the posterior end. On 
the inner costal wall the fleshy attachment reaches to the roots of the very shortened 
forwardly median-directed cartilages (see figs. 37 and 44 respectively). Posteriorly the 
diaphragm forms two short crura and pillars. The external, broader one of these crosses 
outwards from the body of the last dorsal to the under surface of the hindmost rib, 
partially covers the diminutive representative of quadratus lumborum, but debars 
entrance of the depressores caude, though giving egress to, or rather separated by the 
abdominal aorta. The diaphragm at this point folds somewhat on itself, and, fastened 
to the three last ribs, but not to their tips, constitutes a pocket, by the intrusion, so to 
say, of the thick fold of the external oblique muscle. The csophageal and caval 
apertures of the diaphragm are far forwards and wide apart from the crura and pockets 
in question. 
The deepest layer of the outer wall-muscles of the abdomen, the ¢ransversalis, has 
broader and stronger fleshy fibres than the internal oblique, though less so than those 
of the external oblique. It has origin in a digitate manner by attachments from the 
inner surfaces of all the ribs but the second—extending inwards, and joining the 
diaphragm an inch to the vertebral side of the rib-cartilages, and rather more than 
that on the five posterior ribs. The muscular substance extends inwards to near 
the outer third of the rectus, or to within about three inches of the middle line (fig. 9, 
Tra), that measurement being equivalent to its inner aponeurosis. 
A cross section of the abdominal wall shows that the fleshy part of the transversalis 
and of the rectus are coequal ; each muscle thus viewed has a flattened biconvex shape, the 
. Op. cit. p. 428. 
