DR. J. MURIE ON THE FORM AND STRUCTURE OF THE MANATEE. 155 
rectus, as aforesaid, slightly overlapping the transversalis. The peritoneum and a slight 
layer of fat cover these muscles internally ; and it is noticeable that the fat is thickest at 
the median line and rib-cartilages. Between the transversalis and the internal oblique 
muscles is the rete mirabile (Ad. Rete, fig. 9), which lies parallel to the inner fleshy edge 
of the former. é' 
The superincumbent layer of the internal oblique is narrow and shallow. Like the 
external oblique, presently to be described, it has a digitate appearance externally, and 
arises by muscular detachments from the third to the last rib; posteriorly the origins 
are in close contact with those of the external oblique but anteriorly. The fleshy part 
of the muscle extends inwards to the edge of the rectus, and ends in a strong glistening 
tendinous fascia; this passes chiefly over the surface of the rectus to the linea alba, or 
forms the anterior sheath of that muscle, which joins and is lost in that of the external 
oblique. The said aponeurosis is here and there arranged in stronger bundles; and the 
whole has a forward and inward direction contrary to that of the external oblique. 
The external oblique muscle of the abdomen manifestly differs in appearance from 
that of ordinary mammalia in not being spread in a thin uniform sheet over the whole 
abdomen, but rather may be said to be relatively narrow, thick, and composed of a 
series of elongated digitations clasping the extremities of the ribs. Thus it has origin, 
in the manner indicated, from the costze above the cartilages, from the third to the last 
rib inclusive. Its coarse fibres in broad bundles, which mask or imitate separate digitate 
parts, pass inwards and backwards, and end in or are inserted by strong slips of fascia 
into the outer border of the superficial surface of the rectus. These tendinous fibrillz 
are moreover continued in broadish strips over the surface of the rectus, parallel to the 
direction of the fleshy fibres. These latter, it may further be observed, with the oblique 
direction spoken of, proceed from one rib back towards the second behind it—that 
is, embrace three ribs. Posteriorly the external oblique tapers in a wedge-shape 
manner; and this part is inserted upon the surface of the inferior caudal muscle, 
and besides, by the intermuscular fascia, ‘deeply between the last muscle and the long 
tapering muscle coming from the median side line of the ribs. Finally, the wedge 
terminates opposite the second chevron bone. 
In the Sirenian under consideration the rectus abdominis comports in most respects 
with the condition of this muscle in the Cetaceans, e.g. Globiceps and the Porpoise. 
Throughout its entire length, however, it retains its breadth more than in the Whale 
tribe. This may be partly owing to the body of the latter narrowing more posteriorly, 
or partly, no doubt, to the muscle itself serving mesially as a chief support of the 
visceral organs in the land-waddling Manatee, the ribs of the latter being widely apart 
compared with those of the former group. The rectus has origin forwards by a very 
strong tendinous fascia from the outer edge of the sternum, from its projection to the 
ensiform cartilage, and by some sparse muscular fibres from the first, second, and third 
ribs and their sternal cartilages. At this place the pectoralis minor is immediately 
