CETACEA. 



569 



ness and thinness, principally in those at- 

 tached to the atlas and the axis, are extreme; 

 and although those which proceed from the 

 other cervical vertebrae may be better charac- 

 terized, their action, nevertheless, is not much 

 more extensive. 



The muscles of the back present no other 

 important modifications than their great deve- 

 lopment and their prolongation even upon the 

 coccygeal vertebrae. Thus the longissimus dorsi 

 and the sacro-lumbalis are attached anteriorly 

 to the skull, and posteriorly transmit their ten- 

 dons, the first to the end of the tail, the second 

 to all the transverse processes of this part of 

 the spine, associating in this way the move- 

 ments of the back with those of the tail. As 

 to the muscles peculiar to the tail, besides those 

 which belong to this organ in all Mammals 

 where it exists as a moveable organ, there are 

 besides, in the Cetaceans, 1st, the antagonists 

 of the sacro-lumbalis below the transverse pro- 

 cesses ; 2nd, a levator caudte, which takes its 

 rise above the five or six dorsal vertebrae, under 

 the longissimus dorsi, and often in this part 

 blends with it ; it then extends freely as far as 

 the extremity of the tail, where the two muscles 

 unite together again by their tendons ; 3rd, a 

 depressor caudte, of great thickness, which pro- 

 ceeds from the pectoral region, and spreads its 

 tendinous processes upon the ribs, distributes 

 them laterally to the transverse processes, and 

 below to be inserted into the chevron bones 

 along the two posterior thirds of the tail; 4th, a 

 muscle which comes from the rudimental bones 

 of the pelvis, and is inserted into the chevron 

 bones of the anterior portion of the tail ; 5th, 

 the great recti muscles and the obliqui ascen- 

 dentes, which, proceeding from the abdomen, 

 attach themselves behind to the sides of the 

 base of the tail. 



It is in consequence of this great aggre- 

 gation of muscles, which are developed in 

 unexampled proportions as compared with 

 other Mammals, that the tail of the Cetaceans 

 acquires the prodigious strength which it pos- 

 sesses, and by means of which these gigantic 

 animals propel themselves with so much faci- 

 lity and impetuosity through the water, and 

 so readily ascend to the surface to respire, 

 and again seek protection in the deep abysses 

 of the ocean. 



The sternum (q,.fig. 246) is short and large. 

 In the Dugong it is composed of five pieces ; 

 in the Dolphin, the Porpesse, and the Pla- 

 tanist, it is generally composed of only three ; 

 in the Whales it consists of but one. In the 

 subjoined figure (jig. 254) from the Bal<e- 



noptera Boops, the 

 Fig. 254. sternum is deeply 



notched behind, and 

 has a large ridge on 

 its exterior or under 

 surface. 



The ribs of the 

 Cetaceans arechiefly 

 remarkable for their 

 great curvature, but 

 differ in their rela- 

 tive length, thickness, and mode of connection. 

 VOL. r. 



Their thickness and the density of their tex- 

 ture is most remarkable in the Herbivorous 

 species, especially in the Manatee. In the 

 Dugong, which has eighteen pairs of ribs 

 (r, r, Jig. 246), only the first three have car- 

 tilages which join the sternum. In the Del- 

 phinida the first pair of ribs are articulated at 

 their sternal extremities to the anterior angles 

 of the first bone of the sternum ; the second 

 pair join the sternum between the first and 

 second bones ; the third between the second 

 and third, and the fourth, fifth, and in some 

 species the sixth pairs of ribs are joined to the 

 third bone of the sternum ; the sternal portions 

 of these ribs are ossified. The anterior ribs 

 are articulated at first by a head to the ver- 

 tebral centres, and by a tubercle to the trans- 

 verse processes ; but as they extend backwards 

 the head disappears, and the ribs are attached 

 only to the extremities of the transverse pro- 

 cesses. 



In the Baltenida the first pair of ribs are 

 remarkable for their great breadth, especially 

 at the sternal extremity, and these alone join 

 the sternum. In the Baltena Capensis the two 

 first, as well as the four last pairs of ribs, are 

 joined only to the transverse processes of the 

 vertebrae. 



The depressors and elevators of the ribs ap- 

 pear to possess nothing particular, and the 

 same may be said of the diaphragm and the 

 muscles of the abdomen ; but in regard to the 

 movements of these parts, we must remember 

 what M. Mayer says of the muscular fibres, 

 which encircle closely the lungs, and which 

 take part in the actions of inspiration and 

 expiration. 



[Mr. Hunter observes that, " as the ribs in 

 this tribe do not completely form the cavity 

 of the thorax, the diaphragm has not the same 

 attachments as in the Quadruped, but is con- 

 nected forwards to the abdominal muscles, 

 which are very strong, being a mixture of 

 muscular and tendinous parts. The position 

 of the diaphragm is less transverse than in the 

 Quadruped, passing more obliquely back- 

 ward and coining very low on the spine, 

 and high up before, which makes the chest 

 longest in the direction of the animal at the 

 back, and gives room for the lungs to be con- 

 tinued along the spine."] 



The anterior members in the Cetaceans do 

 not essentially differ from those of the other 

 Mammalia, but they undergo, in these animals, 

 very great modifications. 



In the shoulder they are entirely devoid 

 of clavicles. Their scapula is very large in 

 general, but varies in this respect according to 

 the species. In the Herbivorous Cetaceans, as 

 the Dugong (s, Jig. 246), the anterior angle is 

 rounded, the posterior is extended backwards, 

 and the posterior margin or costa is concave. 

 The spine is prominent, and so placed as 

 to divide the dorsum of the scapula into a 

 supra-spinal and infra-spinal depression. The 

 acromion is pointed, but much less elongated 

 in the Dugong than in the Manatee. The 

 coracoid process is also more pointed in the 

 Dugong. 



2 P 



