146 WHALES 



revealed that these whales also lacked a bronchial valve system. The fact 

 that Bottlenose Whales, Sperm Whales and Rorquals, all of them deep 

 divers, share this characteristic deficiency may well indicate that this par- 

 ticvilar lung structiu'e has certain advantages during diving. 



Another striking characteristic of all Cetaceans is the presence of a 

 thick septum between adjoining alveoli, instead of the thin septum of 

 terrestrial mammals (Fig. 84). In this way the resilience of the lungs and 

 the resistance to external pressure fluctuations of the thin alveolar walls 

 are greatly increased. Moreover, every septum can now hold two layers 

 of capillaries, instead of the one found in terrestrial mammals. While the 

 capillaries seem to be embedded in the epithelium cells in much the usual 

 way, the double layer does, of course, provide for more rapid gaseous 

 exchanges with the alveoli, and this is a great advantage, particularly 

 during inhalation at the sui"face. The thick septum leads to the optical 

 illusion that Cetacean alveoli are extraordinarily small, but closer mici'o- 

 scopic observation reveals that they are comparable in size to those of 

 other mammals. Experimental work on the total number of alveoli and 

 the surface area of the epithelium is still too incomplete for any further 

 conclusions to be drawn. 



Another interesting topic, about which too little is still known, is the 

 structure of the blood vessels in Cetacean lungs. In fact, only two French 

 investigators, Lacoste and Baudrimont, have really gone into this question. 

 They discovered, inter alia, that the smaller arteries suddenly branch out 

 into a network of still finer arteries, which twist and turn on their 

 course. Similar phenoinena occur also in other organs whose form can 

 change radically, e.g. the spleen. Thus it may be taken that the pheno- 

 menon is an adaptation to the marked changes in volume which the lungs 

 undergo during respiration, changes which, as we have seen, are far more 

 intense than in terrestrial mammals. Another adaptation is the presence 

 of an annular valve system in the blood vessels, very reminiscent of that 

 in the bronchioles. The two French investigators thought at first that these 

 'valves' occurred in the arteries, but on closer examination Baudrimont 

 (1955) discovered that, in fact, they occurred in the veins. In any case, 

 they, too, are undoubtedly an adaptive mechanism to quick changes in 

 pressure. Additional evidence for this assumption is the fact that each 

 blood vessel is surrounded by a sheath of lymph vessels, which probably 

 acts as a shock-absorber during sudden changes of volume in or round the 

 vessel. 



So far we have only dealt with the structure of the trachea, the lungs 

 and the thorax. Now we must briefly examine the remaining parts of 

 the respiratory system: the larynx, the pharynx, and the nose. The larynx, 



