ALIMENTARY SYSTEM. 649 



The medulla oblongata lies beneath and behind the cerebellum, and 

 is continued into the spinal cord. The cavity of the fourth ventricle is 

 roofed by a thin membrane or velum, above which lies the cerebellum. 

 On the ventral surface the medulla is marked by a deep fissure, bordered 

 by two narrow bands or ventral pyramids. 



The spinal cord presents its usual appearance, with its dorsal sensory 

 nerve-roots with ganglia, its ventral motor nerve-roots apparently with- 

 out ganglia, and the spinal nerves formed from the union of these. The 

 ganglia of the adjacent sympathetic system perhaps belong to the ventral 

 roots of the spinal nerves. 



A large number of nerves pass down the neck. Of these the follow- 

 ing are most important : — 



1. The eleventh cranial nerve or spinal accessory, leaving the skull 



with the ninth and tenth, and distributed to the muscles of the 

 neck. 



2. The twelfth cranial nerve or hypoglossal, lying at first close to 



the ninth, tenth, and eleventh, turning, however, to the muscles 

 of the tongue. 



3. The tenth cranial nerve, the pneumogastric or vagus, lies outside 



the carotid artery, and gives off a superior laryngeal to the 

 larynx with a depressor branch to the heart, an inferior or 

 recurrent laryngeal, which loops round the subclavian artery 

 and runs forward to the larynx, and other branches to the heart, 

 lungs, and gullet. 



4. The cervical part of the sympathetic, lying alongside of the 



trachea, with two ganglia. 



5. The great auricular, a branch of the third spinal nerve, running 



to the outer ear. 



6. The phrenic nerve, a branch of the fourth cervical nerve, with a 



branch from the fifth and sometimes from the sixth, runs along 

 the backbone to the diaphragm. 



For details as to these nerves, the student should consult the practical 

 manuals of Marshall and Hurst and of Parker. 



As to the sense organs little need be said, for their general structure 

 is like that of other Vertebrates, while the detailed peculiarities are 

 beyond our present scope. 



The third eyelid is well developed. The lachrymal gland (absent in 

 Cetacea) lies under the upper lid, and the lids are kept moist by the 

 secretion of Harderian and Meibomian glands. The external ear or 

 pinna is conspicuously large. The cochlea of the inner ear is large and 

 spirally twisted. The nostrils are externally connected with the mouth 

 by a characteristic cleft lip. The tongue bears numerous papillae with 

 taste bulbs. The long hairs or vibrissa? on the snout are tactile. 



Alimentary system. — In connection with the cavity of 

 the mouth we notice the characteristic dentition, the hairy 

 pad of skin intruded in the gap between incisors and pre- 

 molars, the long and narrow, in part bony, palate separating 

 the nasal from the buccal cavity, the muscular tongue with 

 its taste papillae,, the glottis which leads into the windpipe, 



