DE. J. MIJEIE ON THE FORM AND STRUCTURE OF THE MANATEE. 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 (Md, fig. 11). 



It may be remarked of the temjjoralis 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. 



Tliere 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 tendmous. 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. Tims tlie linear arrangement of the upper and 

 loAver 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 GloMceps and 

 Lagenorhynchus I have considered this muscle to be single, as does Stannius* in 

 Phoccena, 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 : Thoracic— 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, pis. 276, 287, h. ' Rccueil, pi. 276,j,j\ 



' i. c. pp. 4 & 5. 4 Phil. Trans. 1868, p. 323. 



VOL. VIII. — PART III. September, 1872. 2 a 



