348 THE ANATOMY OF YERTEBEATED ANIMALS. 



smootli membrane. The posterior diverticulum lies between 

 the posterior valve and the ethmoid and nasal bones. It is 

 incompletely divided by a sort of shelf, is prolonged forward, 

 round, and in front of, the anterior valve, and ends blindly in 

 the middle line above the anterior sac. The spiracular chamber 

 itself is produced, on each side, into a large lateral sac, the 

 walls of which are raised in strong parallel ridges, and covered 

 with a black papillose integument. The walls of these sacs 

 are strong and elastic. Layers of muscular fibres pass from 

 the occipital ridge to the posterior lip of the spiracle, and 

 from the edges of the maxillre to its anterior lip. Their ac- 

 tion is necessarily to open the spiracle and compress the sacs. 

 There is no sphincter, the form of the spiracle causing it to be 

 naturally shut by the fitting together of its walls, and the 

 pressure of the water upon them. 



When a Porpoise comes to tlie surface to " blow," the 

 shape of the posterior, concave lip of the crescentic sj^iracle 

 does not sensibly alter ; but the anterior, convex lip is pulled 

 downward and forward, its surface becoming somewhat de- 

 pressed, and its free edge nearly straight — so that the aperture, 

 when fully dilated, assumes the form of a half-moon. At the 

 same time, the air is exjDelled with a rushing sound. The 

 inspiratory act must be very rapid, as the spiracle remains 

 open for only a very short time alter expiration ends. When 

 the larger Cetacea come up to breathe, the expired vapor 

 suddenly condenses into a cloud ; and, if expiration commences 

 before the spiracle is actually at the surface, a certain quantity 

 of spray may be driven up along with the violent current of 

 the expelled air. This gives rise to the appearance termed 

 the " spouting " of Whales, which does not arise, as it is 

 commonly said to do, from the straining off of the sea-water 

 swallowed with the food, and its exjiulsion by the nostrils. 



The epiglottis, in front, and the arytenoid cartilages be- 

 hind, are prolonged into a tapering tube, dilated at its summit 

 into a knob. Tiie muscular soft palate embraces the neck of 

 this knob so closely that it cannot be withdrawn without 

 considerable effort. And thus, during life, the nasal air- 

 passages and the glottis are kept perfectly continuous ; while 

 the Porpoise dashes through the water, open-mouthed, after 

 its prey. The point at which the extra bronchus to the right- 

 lung is given off is separated by four rings from the bifurcation 

 of the trachea. The lungs are not lobed and their tissue is 

 very dense and elastic. 



The cerebral hemispheres are, taken together, broader tlias 



