578 



CETACEA. 



The disappearance of the posterior members 

 has occasioned that of the vessels which should 

 nourish those members ; and as the tail has 

 attained a considerable development, the arte- 

 ries and veins which belong to this last part of 

 the trunk have been developed in the same 

 proportion. The abdominal aorta does not 

 send off any external iliacs, but is continued 

 underneath the tail in the canal of the inferior 

 processes, from whence its ramifications are dis- 

 tributed to the muscles which move this organ. 

 The modifications of the venous system are in 

 many respects analogous to those of the arteries. 



Fig. 266. 



The quantity of blood contained in the vascu- 

 lar system appears to be proportionally much 

 greater than in the other Mammalia. 



[In the Porpesse the veins are almost univer- 

 sally devoid of valves, so that they can be as 

 easily injected from trunks to branches, as in 

 the reverse direction. The plexiform disposi- 

 tion which we have seen to characterize so 

 many parts of the arterial system is still more 

 strongly displayed in the venous. Thus in the 

 system of the anterior vena cava, with the ex- 

 ception of the trunk of that vein itself, and the 

 short jugular veins which join it, an internal 

 and an external jugular branch, and a pair of 

 large subcutaneous veins, all the other parts of 

 the system manifest the plexiform 

 disposition. This is most remark- 

 able in the large venous sinuses 

 surrounding the central axis of the 

 nervous system, which receives the 

 intercostal veins, and by means of 

 which the system of the anterior 

 cava is chiefly brought into com- 

 munication with that of the pos- 

 terior cava ; for, as V. Baer has 

 observed, there is no intercommu- 

 nicating channel analogous to the 

 vena azygos of the higher Mam- 

 malia. 



Of the venous plexuses belong- 

 ing to the system of the inferior 

 cava, that which is found at the 

 posterior parietes of the abdominal 

 cavity extending from below the 

 kidney to the lower boundary of 

 the abdomen is the most remark- 

 able, and we have selected in 

 illustration of this, the figure from 

 Baer's excellent memoir on the vas- 

 cular system of the Cetacea.* In 

 this figure (Jig. 266) the anterior 

 parietes of the abdomen are remo- 

 ved. The two immense lateral de- 

 pressor muscles of the tail are seen 

 at A, A, and B shows their point 

 of convergence to be inserted into 

 the inferior spinous processes, by 

 which the cavity of the abdomen is 

 contracted and defined posteriorly. 

 Just anterior to this commissure 

 is seen the termination of the rec- 

 tum H. C, C, are the two ischia. 

 D, D, the posterior parietes of the 

 chest projecting forwards over the 

 abdomen. On the right side the 

 kidney and the peritoneum are re- 

 moved; on the left side they are 

 seen in situ, and also a part of the 

 left cornu of the uterus G, with 

 the oviduct and ovary K. 



At p is seen the inferior vena 

 cava cut through, which lies in the 

 interspace of the two great depres- 

 sors of the tail. The trunk of the 

 vena cava seems smaller than it 



Abdominal venous plexus and kidney of the Porpesse. 



* Ueber das Gefass - system des 

 Braunfisches, Nova Acta, Phys. Med. 

 Leopold. Carol, torn. xvii. 1835. 



