MUSCULAR SYSTEM OF MAMMALIA. 



the shore bv means or aid of their anterior members, which in the 



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true Cetacea are exclusively natatory organs. 



The head, in these, has so little mobility, that its axis can be 

 but slightly altered, without that of the body altering also. With 

 bones so short, so little mobile, and extensively co-adapted or 

 anchylosed, as the vertebrae of the neck, muscles proportionately 

 reduced should correspond. The cervical muscles are, neverthe- 

 less, much the same in number as in other Mammals ; but their 

 shortness and thinness, principally in those attached to the atlas 

 and the axis, are extreme. The homologue of the ( splenius 

 capitis,' fig. 10, h, is the best developed: it comes off from the 

 anterior dorsal and cervical series of neural spines, and its fibres 

 converge to be inserted into the paroccipital ridge. 



The muscles of the back present no other important modifica- 

 tions than their great developement, especially where they are 

 prolonged upon the caudal vertebras. Thus the longissimus dor si 

 and the sacro-lumbalis are attached anteriorly to the skull, and 

 posteriorly transmit their tendons, the first to the end of the tail, 

 the second to all the transverse processes of this part of the spine, 

 associating its movements, especially in the vertical direction, with 

 those of the back. The levator caudcs, takes its rise above the five 

 or six dorsal vertebra?, under the longissimus dorsi, and often in 

 this part blends with it ; it then extends freely as far as the ex- 

 tremity of the tail, Avhere the two muscles unite together again by 

 their tendons. They are opposed by a depressor caudce, of great 

 thickness, which proceeds from the thoracic region, attached by 

 tendinous slips to the ribs and the contiguous transverse processes ; 

 it is inserted into the haemal arches of the tail. A muscle passes 

 from the rudimeiital bones of the pelvis to the haamapophyses of 

 the anterior portion of the tail. The great recti abdominis and 

 obliqui ascendentcs muscles are continued backward from the ab- 

 domen, and attach themselves behind to the sides of the anterior 

 caudal vertebrae. By this aggregation of muscles the tail of the 

 Cetacea expands to proportions of the trunk, and acquires the 

 prodigious strength which it possesses for propelling the most 

 gigantic of the species, with ease and swiftness, through the 

 water ; and, by means of the horizontal expansion of the caudal 

 fin, it enables them to readily ascend to the surface to respire and 

 again seek protection in the deep abysses of the ocean. 



In the great pectoral muscle, part of which is shown in fig. 10, 

 at g, the costal origin is extensive, and the portion which comes 

 off from the short sternum, passing transversely each to its own 



