CETACEA. 



579 



really is, on account of its deep position and 

 the overlapping of the kidney, E. As it gets 

 beyond this part it is seen to dilate. Two veins, 

 corresponding to the vena iliacee of Quadrupeds, 

 (m, in,) return the blood in part to the tail, and 

 join the vena cava near the kidneys. The vein 

 corresponding to the caudal or sacro-median of 

 Quadrupeds is not a simple vessel, but a 

 plexus, which is surrounded and protected by 

 the inferior spinous processes ; it is seen at_/'. 

 A venous plexus from the intestinal canal (g) 

 terminates in the right iliac vein, which is 

 larger than the left, and thus establishes a com- 

 munication between it and the portal system. 

 h shows a muscular vein, and i the termination 

 of a hypogastric plexus. 



The more important plexuses which commu- 

 nicate with the iliac veins are, first, the perito- 

 neal plexus (/), which in older individuals, and 

 especially at the season of sexual excitement, is 

 much more considerable than is here repre- 

 sented ; and secondly, the iliac or psoadic 

 plexus (k,k), which forms an immense reser- 

 voir of venous blood. It is situated between 

 the under surface of the depressors of the tail, 

 which represent the psoas muscles, and the 

 peritoneum, reaching from behind the lower 

 extremity of the kidney to the posterior end of 

 the abdomen, and forming a mass of closely 

 interwoven veins, of an inch or more in thick- 

 ness, and serving to bring the subcutaneous 

 veins of the posterior part of the body into 

 communication with the posterior vena cava. 



This plexus is fed, if we may use the ex- 

 pression, by , an inferior vein; b, a lateral; 

 and r, a superior vein of the tail, which unite 

 to form an ischiadic sub-plexus, d. Laterally 

 the iliac plexus receives from five to seven 

 veins, which return the blood from the dorsal 

 and lateral parietes of the abdomen, and pierce 

 the lateral abdominal muscles to join the plexus 

 at e, e. On its internal or mesial edge the iliac 

 plexus communicates by many and wide aper- 

 tures with the iliac vein. At the anterior part 

 of the abdomen the inferior cava receives the 

 plexus phrenicus, <>, o. 



The condition of the venous system above 

 described, while it is admirably adapted to the 

 mode and sphere of existence of the Cetaceans, 

 presents a beautiful instance of that co-ordinate 

 analogy to the condition of the veins in the 

 embryo of the higher Mammals, which is ex- 

 hibited in the general form of the animals 

 composing this the lowest order of the class.] 



ORGANS OF RESPIRATION. The organs and 

 all the essential phenomena of respiration are 

 the same in the Cetaceans as in the other 

 Mammals. They have been made the subject 

 of but few observations. 



[In the Dugong the lungs are of a very elon- 

 gated and flattened form, resembling those 

 which Uaubenton has figured of the Manatee. 

 They are, as Sir Everard Home has observed, 

 one-fourth the length of the animal ; those 

 from the animal, eight feet long, which he re- 

 ceived from Sir Stamford Raffles, measuring 

 two feet. They are convex posteriorly or on 

 the dorsal aspect, flattened on the opposite 

 side, and along this surface the principal 



branches of the bronchi can be seen through 

 the serous covering. The upper end of each 

 lung is obtuse, thick, and narrow; they gradu- 

 ally become flatter towards the lower extremity, 

 the margin of which is rounded. 



The whole surface of these lungs presents an 

 appearance somewhat similar to that of the 

 Turtle (Chelonia Mydas), in consequence of 

 the large size of the superficial air-cells, which 

 are a line in diameter (a, a, fig. 268.) The 

 great extent of the lungs down the back, and 

 the high division of the trachea, and consequent 

 length of the bronchi, are further instances of 

 this resemblance. 



Fig. 267. 



Cartilages of ttte bronchus of the Dugong. 



The cartilages of the bronchial tubes are 

 continued spirally into one another (fig. 267) : 

 the pulmonary artery lies to the outer side of 

 the bronchus and is deeper seated; the pulmo- 

 nary vein to the inner side, and is superficially 

 situated. The principal branch of the bron- 

 chus (b, fig. 268) runs down near the inner 

 margin of the lung, and continues distinct to 

 within four inches of the end ; it then divides 

 into smaller branches; the larger ramifications 

 are given off from its outer side, c, c. In all 

 the branches the cartilaginous rings continue 

 distinct and strong till their diameter is con- 

 tracted to one or two lines ; the rings passing 

 irregularly into each other as in the main 

 trunks. The lining membrane of the air- 

 tubes is thrown into longitudinal rugae, in- 

 dicating their dilatability. We have before 

 mentioned the large size of tie pulmonary 

 artery : in this respect, as well as in the 

 structure of the lung, the Dugong manifests 

 a greater similarity to the reptile than the 

 Porpoise does. In this animal the air-cells in 

 no part of the lung exceed a sixth part of the 

 size of the superficial ones in the Dugong; and 



