lyÖ WHALES 



can safely deduce a particularly close relationship to these mammalian 

 orders. This is further evidence in support of the assumptions we have 

 made on this subject in Chapter 2. The extremely small size of the spleen 

 may be an indication that Cetaceans can largely dispense with those 

 substances in that organ which regulate blood pressure and respiration. 

 It may be assumed that, because of their special method of respiration and 

 their diving habits, this regulation is effected in a different way, with the 

 proviso that our remarks about the spleen apply only to Cetaceans and 

 not to aquatic animals in general. The spleens of seals and sea-lions, for 

 instance, represent an average of 0-45 per cent of the animals' total 

 weight, a much higher figure than that for terrestrial mammals. Hence the 

 small size of the Cetacean spleen may well be associated with the presence 

 of retia mirabilia, which are absent or poorly developed in seals and sea- 

 lions. Since the retia play an extremely important part in regulating the 

 blood pressure, it is quite possible that they have relieved the spleen of 

 much of its normal work. 



The reader might wonder whether the diminutive size of the Cetacean 

 spleen also vitiates its ability to break down worn red blood corpuscles and 

 to defend the body against bacterial infection. Unfortunately, we cannot 

 give a definite answer to this question. All that is known is that Cetaceans 

 have an exceptional number of very large lymph glands, which are another 

 mammalian line of defence against microbes, and another means of break- 

 ing down worn-out red blood cells. When examining the lymph glands of 

 some dolphins under the microscope I saw clear signs of such a break- 

 down, and it is quite possible that, in Cetaceans, the lymph glands have 

 to some extent taken over this function from the spleen. 



So much for the Cetacean spleen, and if what we have said seems some- 

 what inconclusive, the zoologist's only excuse is that his knowledge of the 

 human spleen is not much greater. 



In conclusion we might add that Cetaceans have tonsils, like all other 

 mammals, and in the same place. Little else can be said about them, since 

 this mass of lymphatic tissue has scarcely been examined in Cetaceans. 

 The same may be said of the thymus gland, a dark red lobular organ 

 found in the anterior thorax of foetuses and young animals. In Cetaceans, 

 as in all other mammals, the thymus begins to atrophy with the onset of 

 puberty, after which it gradually disappears. We mention the thymus 

 merely because it, too, consists of lymphatic tissue. Little else is known 

 about its function, though English physiologists have recently discovered 

 that, in man, the thymus may be associated with the pathological condi- 

 tion known as myasthenia gravis, the main symptoms of which are extreme 

 fatigue and serious muscular impairment. If the respiratory muscles are 

 affected, this condition may have fatal consequences. Apparently, 



