DR. J. MURIE ON THE FORM AND STRUCTURE OF THE MANATER. 151 
is covered in great part by the facial portion of the panniculus; but its fibres are quite 
separate, and differ in direction from those of the panniculus. It passes downwards 
and forwards between the depressor anguli oris and the depressor labii inferioris, termi- 
nating among the fibres of the former and upon the surface of the latter. It appears to 
counteract and check the action of the two previously mentioned muscles. Provisionally 
I name it mandibularis (Ma, fig. 11). 
It may be remarked of the temporalis that, considering the great size of the bones 
and capacity of the temporal fossa, it is relatively small, and covered with a great mass 
of fat. The temporal muscle, of fair size in the Elephant’, is upwardly elongate, as is 
the skull; but in Whales it is the reverse of this, being set obliquely backwards in 
direction, short and thick. 
There is a double masseter. The broadest and strongest portion, relatively weak in 
itself, is that which, fan-shaped, and with an obliquely forward and upward direction, 
stretches from the outer surface of the broad mandibular angle to the descending process 
of the malar arch, where it is most strongly tendinous. ‘The narrower deeper portion 
or layer is attached to the ascending ramus and to the hinder half and lower border of 
the malar arch. The fibres of this portion run counter to the upper layer; that is, they 
assume a downward and forward course. Thus the linear arrangement of the upper and 
lower muscular fibres is contrariwise or x-shaped, the diagonal of the forces between 
which necessarily acts in an up-and-down direction. 
The facial artery, nerve, and Stenon’s duct, as usual, cross the masseter, but parallel 
to each other, and in a nearly horizontal line. These and the muscle are entirely 
covered by the thick extension forwards of the panniculus carnosus. 
In Elephas the masseter unequivocally has two layers’; the fibres, however, are more 
nearly alike, and vertical, than in the Manatee. In my own dissection of Globiceps and 
Lagenorhynchus 1 have considered this muscle to be single, as does Stannius® in 
Phocena, with some additional fibres which he terms malaris externus; but in the 
Pike Whale* the masseter is stated to consist of two planes of fibres, superficial and 
deep. 
(C) Those of the Costal Arches: Thoracie—The intercostal muscles, seventeen in 
number on each side, are remarkably strong and fleshy. The external series are by 
far the thickest of the two. They are oblique in direction, but not so much as the 
internal series. . The diminished length of the costal cartilages causes the above muscles 
to stop short wide of the median line of the abdomen; but, with this exceptional circum- 
stance, they agree with their ordinary situation and attachments. 
There is an arterial intercostal plexus, as Stannius has noted, betwixt the various ribs. 
This at first lies within the external intercostal muscle, covered by the pleura costalis, 
then dips between the internal and external intercostales. 
? Recueil, pls. 276, 287, b. * Recueil, pl. 276, j, 7’. 
5 L.c. pp. 4 & 5. * Phil. Trans. 1868, p. 323. 
VOL. VII.—PART 11. September, 1872. 2A 
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