(2362). The interior of the nasal cavity is divided by a median septum into chambers, in each of which a very large surface is produced by the complicated convolutions of the thin nasal plates of the ethmoid (fig. 406, a), and of the inferior turbinated bone (6), over which the air is made to pass in its progress to the lungs before it arrives at the posterior nares (c.) The whole of this complication of bony lamellae is covered with a delicate and highly-lubricated mucous membrane, wherein the olfactory nerves terminate; and from the figure given, representing the left nasal cavity of a Lion, some idea may be formed of the acuteness of the sense in question conferred upon the predaceous Carnivora.

(2363). With this perfection of the olfactory sense a corresponding-mobility of the outer nostrils is permitted to the Mammiferous races. In the Reptiles and Birds the external apertures leading to the nose were merely immovable perforations in the horny or scaly covering of the upper mandible; but now the nostrils become surrounded with moveable cartilages and appropriate muscles, adapted to dilate or contract the passages leading to the nose, or even to perform more important and unexpected duties, as, for example, in the proboscis of the Elephant.

(2364). The Cetacea, as regards the conformation of their nostrils, and indeed of the whole of their nasal apparatus, form a remarkable exception to the above description. Inhabiting the water as these creatures do, they are compelled to breathe atmospheric air. Are they, then, to smell through the intervention of an aquatic or aerial medium?

To smell in water would require the nose of a fish, which could not be granted without infringing upon the laws that regulate the progression of animal organization. To smell in air would be useless to the Whale; and moreover its nasal passages are required for another function, with which the exercise of smell would apparently be incompatible.

(2365). Thus circumstanced, we find the whole nasal apparatus completely metamorphosed, and so disposed as to answer two important purposes: viz. first, to allow the Cetacean to breathe air whilst its mouth is immersed in water; and second, to provide an outlet whereby the water that is necessarily taken into the mouth may escape without being swallowed.

(2366). The arrangement adopted to attain both these ends is very beautiful. The nostrils, instead of occupying their usual position, are situated quite upon the top of the head (fig. 407, a); so that as soon as the vertex reaches the surface, air is freely obtained. But another difficulty remains to be overcome: how is the Cetacean to breathe air while its mouth is full of water?

(2367). To allow this, the upper extremity of the larynx is prolonged so as to form a thick cartilaginous plug (c.) When the creature breathes, this elongated larynx is introduced into the posterior nares, as represented in the figure; and, being firmly embraced by a sphincter muscle whilst in that situation, the air is admitted into the trachea through the passages (a, b), without ever entering the oral cavity.

Blowing apparatus of the Porpoise.

Fig. 407. Blowing apparatus of the Porpoise.

(2368). 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.

(2369). 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 chambers are enveloped by a very strong expansion of muscular fibres, by which they can be violently compressed.

(2370). 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 posterior nares (6), 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.

(2371). 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.

(2372). The second pair of ganglia entering into the composition 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 Eeptile these were at once recognizable as primary elements of the brain; but in the Mammifer, owing to the excessive development of the surrounding parts, they are quite overlapped and concealed by the hemispheres.

Brain of the Rabbit.