OPTIC LOBES OF THE BRAIN. 



plished. The two canals forming the posterior nares (6) 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 com- 

 munication 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 fall, the walls are distended so as to 

 form capacious oval receptacles. Externally these chambers are enve- 

 loped 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 Fig. 408. 



do this, the valve between the pouches 

 and the posterior nares being firmly 

 closed, the sacs are forcibly com- 

 pressed by the muscles that embrace 

 them, and the water is then spouted 

 up through the "blow-holes," or nos- 

 trils, 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 con- 

 tinually 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 Brain of the Eabbit. 



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 Keptile 

 these were at once recognizable as primary elements of the brain ; but 

 in the Mammifer, owing to the excessive development of the surround- 

 ing parts, they are quite overlapped and concealed by the hemispheres. 

 * Cuvier, Leqona d'Anat. Comp. ii. p. 673. 



