140 SKETCH OF THE HISTORY OF MAMMALIA. 



the sides of the thorax, pass in between the ribs near their 

 articulation, and also behind the ligamentous attachment 

 of the ribs, and anastomose with each other. The spinal 

 cord is surrounded with a network of arteries in the same 

 manner, more especially where it comes out from the 

 brain, and where a thick substance is formed by their 

 ramifications and convolutions. Fig. 83 represents the 

 arrangement of the arteries of the chest. 



Nor is this plexiform arrangement peculiar to the ar- 

 terial system ; it is even more strongly displayed in the 

 venous ; the veins in certain parts forming immense 

 plexiform reservoirs. This curious disposition of the 

 vascular system in the Cetacea is, as we have intimated, 

 most probably connected with their habits of diving, 

 during which their respiration is suspended, and conse- 

 cpiently the passage of the blood through the lungs im- 

 peded ; while at the same time they are subjected to a 

 great pressure. Hence perhaps the arterial plexuses are 

 needed as reservoirs stored with oxygenated blood for 

 the use of the system, wdiile the venous plexuses are 

 reservoirs for the sake of safety during the suspension of 

 respiration. 



The females of this order have two teats deeply 

 imbedded at the lower portion of the abdomen. The 

 stomach is very complicated, divided into several 

 compartments, and digestion is very rapid. 



As these animals have to plough the waves head foremost, 

 a long or even decided neck would interfere with their 

 movements, consequently the cervical vertebrse are com- 

 pressed into a small space, and more or less entirely 

 anchjdosed into one mass, to the total immobility of the 

 head, the axis of which cannot be altered without a 

 corresponding alteration of that of the body. It is more- 

 over remarkable that the two halves of the head do not 

 precisely correspond in symmetry. This is especially 

 observable in the dolphins, porpoises, grampus, and 

 cachalot. (See Mechel's ' Anatomic Comparee/ vol. 

 iv. p. 361.) 



The Cetacea are all carnivorous, but their prey differs 

 from the small mollusk to fishes and even the smaller of 



