684 MAMMALIA. 



large part covered by the cerebrum. Between them runs the iter con- 

 necting the third ventricle and the fourth. The floor of this passage is 

 formed by the thick crura cerebri which connect the medulla with the 

 cerebrum. 



The cerebellum has a median and two lateral lobes (with accessory 

 flocculi), and is marked by numerous folds, mostly transverse. The 

 two sides are connected ventrally by the pons Varolii, lying across the 

 anterior ventral surface of the medulla. 



The medulla oblongata lies beneath and behind the cerebellum, and 

 is continued into the spinal coid. 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 

 larnyx 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 



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 papilla with 

 taste bulbs. The long hairs or vibrissrc on the snout arc tactile. 



