800 MAMMALIA. 



(2338.) In the arrangement of the blood-vessels of the CETACEA, 

 many interesting peculiarities are met with *. The general structure 

 of the arteries, indeed, resembles that of other Mammals, and, where 

 parts are nearly similar, their distribution is likewise similar. But 

 these animals have a greater proportion of blood than any others known; 

 and there are many arteries apparently intended as reservoirs, wherein 

 a large quantity of arterial blood may accumulate apparently for im- 

 portant purposes, where vascularity could not be the only object. Thus 

 the intercostal arteries divide into a vast number of branches, which run 

 in a serpentine course between the pleura and the ribs, and penetrate 

 the intercostal muscles, everywhere lining the walls of the thorax. 

 These plexiform vessels, moreover, pass in between the ribs near their 

 articulation, and anastomose extensively with each other. The medulla 

 spinalis is likewise surrounded with a network of arteries in the same 

 manner, more especially as it comes out from the brain, where a thick 

 substance is formed by their ramifications and convolutions ; and these 

 vessels most probably anastomose with those of the thorax. The precise 

 function assigned to this extensive plexus of arteries has not been as 

 yet satisfactorily determined, although it is doubtless a receptacle wherein 

 arterial blood is stored up during the long-continued submersion to which 

 these animals are so frequently subjected. 



(2339.) As the CETACEA have no pelvic extremities, the aorta, instead 

 of bifurcating into iliac arteries, is entirely appropriated to supply the 

 enormous tail, beneath which it is continued, enclosed in a canal formed 

 by the roots of the inferior spinous processes of the caudal vertebra, 

 that are here again developed as in fishes. 



(2340.) The venous system in the Cetacean order is equally remark- 

 able for the plexuses formed by it in different parts of the body; of 

 these, the most important communicates with the abdominal cava, and is 

 of immense extent. The veins of these creatures, moreover, are almost 

 entirely deprived of valves ; so that every possible arrangement has been 

 made to delay the course of the circulating blood during the temporary 

 suspension of respiration that occurs whenever the animal plunges 

 beneath the surface of the water. 



(2341.) In other aquatic Mammals that dive, and are thus subjected 

 to prolonged immersion, large dilatations are found connected with the 

 principal trunks of the venous system in the neighbourhood of the 

 heart, in order to prevent a dangerous distension of these veins while 

 the circulation is impeded and respiration put a stop to. This is parti- 

 cularly remarkable in the Seal tribe ; and in these Carnivora we are 

 assured by good authorities that it is not uncommon to find the foramen 

 ovale of the heart, and the ductus arteriosus, which in the foetus allows 

 blood to pass from the pulmonary artery directly to the aorta, still open 

 even in the adult animal ; but this arrangement, as we are well satisfied, 

 * Hunter, ut supra, p. 365. 



