154 DE. J. MTTRIE ON THE FORM AISTD STEUCTUEE OF THE MANATEE. 



genera of the group, Halicore and Bhytina, 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 PI. XXVI. figs. 44, 

 37, 49, and 50, and also partially in PI. 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 ny 

 at point apergu de centre nerveux," meaning, I apprehend, that there is no ordinary 

 free central tendon ; but instead, as fig. 44, PI. XXVI. 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 caudse, 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 oesophageal 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 transversalis, 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. clt. p. 428. 



