HEART, CIRCULATION, AND BLOOD 1 75 



explain the function of the spleen. The wretched student was extremely 

 discomfited by this unexpected question and finally stammered out that, 

 though he had once known the answer, somehow he had forgotten it. 

 'What a terrible pity,' Claude Bernard rejoined, 'because no one before 

 you has ever known it at all.' 



For a long time, it was generally believed that the spleen, which is part 

 of the circulatory system, contracted or relaxed to decrease or increase the 

 capacity of the circulatory system. Modern opinion, however, is that the 

 spleen plays a negligible part in regulating the blood flow compared with 

 other organs. Biologists now think that the spleen retains some of the blood 

 passing through it for some time, during which the chemical composition 

 of the blood is changed to form substances which regulate the blood 

 pressure, the breakdown of red blood cells and probably respiration as 

 well. Moreover, the spleen also plays a definite role in the body's defences 

 against bacteria and other harmful organisms, with some of its cells 

 destroying them or counteracting their effect. 



Since we know so little about our own spleen it seems strange that we 

 should be discussing that of Cetaceans. In fact, we would have passed 

 over this subject in dignified silence, but for the fact that the Cetacean 

 spleen has a very striking characteristic - it is remarkably small. 



True, the spleen of big Rorquals weighs some 6-22 lb., the average 

 being 13 lb., but that is only about 0-02 per cent of the animal's total 

 weight, as against 0-3 per cent in most other mammals. In other words 

 the spleen of the Rorqual is proportionally ^-g that of most terrestrial 

 mammals, and so is the spleen of many porpoises and dolphins. Aloieov^er, 

 the spleen of Cetaceans does not have a very distinctive shape. In 

 Rorquals, it is generally elongated, some 60 cm. long, and rather narrow 

 and flat, though many other shapes occur as well. Just as in all other 

 mammals, the spleen has a bright reddish-brown colour and is attached 

 to the stomach by a peritoneal fold. Sometimes one or even more small 

 accessory spleens are also present. 



In order to gain a better vmderstanding of the reason why the Cetacean 

 spleen is so small. Miss H. H. L. Zwillenberg, a biology student at 

 Amsterdam University, made a painstaking investigation of their fine 

 structure ( 1 956-7) . She came to the conclusion that there is a charac- 

 teristic difference between the spleen of Odontocetes and that of Mysticetes, 

 as the former have a far greater number of lymph corpuscles, i.e. of white 

 spleen pulp. (In porpoises, for instance, this pulp accounts for 30 per cent 

 of the total spleen content.) This difference is probably due to the two 

 Cetacean sub-orders being descended from terrestrial animals with 

 different types of spleen, though the spleens of both have so many charac- 

 teristics in common with the spleens of Carnivores and Ungulates that we 



