MAMMALIA. 695 



It only remains to be seen how the Cetacean gets rid of the 

 water taken into the mouth, without being obliged to swallow it ; 

 and the same figure, representing a vertical section of the head of a 

 Porpoise, will enable us to understand the mechanism whereby 

 this is accomplished. The two canals forming the posterior nares 

 (b) are defended superiorly by a fleshy valve,* which is closed by 

 means of a very strong muscle placed above the intermaxillary 

 bones. To open this valve the force must be applied from below ; 

 and, when the valve is shut, all communication is cut off between 

 the posterior nares and the capacious cavities placed above them. 



These cavities are two large membranous pouches lined with a 

 black skin, which, when they are empty, as represented in the 

 figure, falls into deep folds ; but, when full, the walls are distended 

 so as to form capacious oval receptacles. Externally these cham- 

 bers are enveloped by a very strong expansion of muscular fibres, 

 by which they can be violently compressed. 



Let us now suppose that the Cetacean has taken into its mouth 

 a quantity of water that it wishes to expel : it moves its tongue 

 and its jaws as though it would swallow ; but, at the same time 

 closing its pharynx, the water is forced upwards through the pos- 

 terior nares (b), till it opens the interposed valve, and distends the 

 pouches placed above. Once in these reservoirs, the water may 

 remain there until the creature chooses to expel it, or in other 

 words " to blow." In order to do this, the valve between the 

 pouches and the posterior nares being firmly closed, the sacs are 

 forcibly compressed by the muscles that embrace them, and the 

 water is then spouted up through the " blow-holes," or nostrils, 

 to a height corresponding to the violence of the pressure. 



(804.) It must be evident that it would be impossible that a 

 nose, through which salt water is thus continually and violently 

 forced,, could be lined with a Schneiderian membrane of sufficient 

 delicacy to be capable of receiving odorous impressions. In the 

 CETACEANS therefore the nerves of smell, and even the olfactory 

 lobes of the brain, are totally deficient. 



(805.) The second pair of ganglia entering into the composi- 

 tion of the encephalon, and giving origin to nerves, are the optic 

 lobes ; from which are derived the nerves of vision. In the Fish 

 and in the Reptile these were at once recognizable as primary ele- 

 ments of the brain ; but in the Mammifer, owing to the excessive 

 developement of the surrounding parts, they are quite overlapped 



* Cuvier, Le9ons d'Anat. Comp. torn. ii. p. 673. 



