DE. J. MUEIE ON THE FORM AND STEUCTUEE OF THE IHANATEE. 155 



rectus, as aforesaid, slightly overlapping the transversalis. The peritonaeum and a slight 

 layer of fat cover these muscles internally ; and it is noticeahle that the fat is thickest at 

 the median line and rib-cartilages. Between the transversalis and the internal oblique 

 muscles is the rete mirabile (Ab.Bete, fig. 9), which lies parallel to the inner fleshy edge 

 of the former. 



The superincumbent layer of the internal oblique is naiTow and shallow. Like the 

 external oblique, presently to be described, it has a digitate appearance externally, and 

 arises by muscular detachments from the thii'd 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 contrai^ 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 costse 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 fibrillee 

 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 naiTowing 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 cartOage, 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 



