202 INTRODUCTION TO NEUROLOGY 



sensory spots in the saccule, demonstrates the relatively late 

 phylogenetic origin of the cochlear system from the vestibular, 

 and has suggested to some physiologists that even in man these 

 two systems are not wholly distinct, and that the sense organs in 

 the saccule may also function as a sound receptor. It is clear, 

 however, that tone analysis is effected only in the cochlea. 



The central connections of the cochlear and vestibular nerves 

 are fundamentally different. The vestibular nerve terminates 

 in reflex centers of the medulla oblongata and cerebellum (p. 

 185) with no important cortical connections, while the cochlear 

 nerve has, in addition to important reflex connections in the ob- 

 longata and midbrain, the much stronger ascending pathway of 

 the lateral lemniscus directly to the medial geniculate body of 

 the thalamus, and thence to the temporal lobe of the cerebral 

 cortex (see p. 157 and Figs. 75, 77, 80, 96). Some of the fibers 

 of the lateral lemniscus are interrupted in the inferior colliculus, 

 which is an important auditory reflex center. 



Summary. The human ear has three parts: (1) the external 

 ear, for receiving sound waves from the air; (2) the middle ear, 

 for intensifying the vibrations and transmitting them to (3) the 

 internal ear, which is filled with liquid and contains sense organs 

 of uncertain function in the utricle and saccule, sense organs for 

 equilibration in the semicircular canals, and the spiral organ 

 (organ of Corti) in the cochlea for tone analysis. The spiral 

 organ is a complicated epithelial structure resting on a basement 

 membrane and consisting of supporting cells of diverse kinds, 

 the hair cells (which are the specific receptors and receive the 

 endings of the fibers of the cochlear nerve), and the tectorial mem- 

 brane. Shambaugh is of the opinion that the tectorial membrane 

 is capable of responding in its various parts to different vibration 

 frequencies, and that the hair cells are stimulated through their 

 hairs which are attached to the tectorial membrane. 



In fishes the organ of hearing is much simpler than in man, the 

 semicircular canals are, however, similar, and there is, in addi- 

 tion, an elaborate system of lateral line sense organs whose func- 

 tions seem to be intermediate between the tactile and auditory 

 organs. It is probable that these three systems of sense organs 

 were derived phylogenetically from some more generalized form 

 of cutaneous tactile organ. This accounts for the intimate as- 



