150 WHALES 



always swallow their food under water, though it is known that young seals 

 must first be taught to do so. When they start feeding on fish, they bring 

 their prey up to the surface where they swallow it, but after about two 

 ^\•eeks they join their parents in swallowing their food below. 



A better explanation might be connected with another characteristic 

 distinction between Odontocetes and Mysticetes : the presence or absence 

 of diverging diverticula or membraneous folds in the upper nasal passage. 

 We shall return to these diverticula later, but here we should like to point 

 out that they can be filled with air during diving, the beak mechanism 

 preventing this air from escaping through the mouth. Moreover, as we 

 shall see, it seems likely that during sti^ong pressure fluctuations these 

 animals expel some air from the trachea. To do so gradually, they need 

 a successive tap system of the kind found in the bronchioles (see above) . 

 This is clearly provided by the beak as the first tap, and the air plug 

 formed by the diverticula in the nostril as the second. 



In most terrestrial mammals the nasal passage, though long, does not 

 have the capacity we might think it has. From the lateral wall of each 

 nasal cavity spring three twisted laminae of cartilage (conchae), dividing 

 each side of the nose into a number of narrow passages (meatuses) . The 

 meatuses are lined with ciliated epithelium which serves as a dust filter, 

 and they contain small glands for keeping the air moist. They are also 

 provided with a network of veins which help to keep the air warm. The 

 posterior part of the uppermost compartment of the nose is provided with 

 olfactory receptors embedded in the mucous niembrane lining the 

 numerous lamellae of the ethmoid bone. 



We have seen that in all Cetaceans, with the exception of the Sperm 

 Whale, the blowhole, i.e. the nostril, is found on top, rather than at the 

 front of the head, so that the nasal passage rises up almost vertically from 

 the throat. The passage is not only relatively short but also relatively wide 

 when open, since conchae are absent in Cetaceans. All this obviously makes 

 for quicker air displacement, and we know that Cetaceans breathe far more 

 violently than we do. In Cetaceans there is no ciliary epithelium, which 

 they do not need, since the air they breathe is free of impurities, and there 

 is also a complete absence of small moisture-producing glands - the air 

 being moist enough as it is. However, the absence of veins in this large 

 nostril is a disadvantage, and is probably compensated for by the presence 

 of a highly developed network of veins in the wall of the trachea and 

 the bronchi which have probably taken over the function of warming 

 the air. 



In Odontocetes, the olfactory receptors (and the olfactory nerve) are 

 entirely absent or present in rudimentary form only. Mysticetes, however, 

 are said to have retained a slight sense of smell (see Chapter 9), for close 



