718 



Or THE FUNCTIONS OP THE NERVOUS SYSTEM. 



A view of the origin and distribution of the Portio 

 Mollis of the Seventh pair or Auditory Nerve : 1, the 

 medulla oblongata ; 2, the pons Varolii ; 3, 4, the 

 crura cerebelli of the right side ; 5, the eighth pair of 

 nerves ; 6, the ninth pair ; 7, the auditory nerve dis- 

 tributed to the cochlea and labyrinth ; 8, the sixth 

 pair of nerves ; 9, the portio dura of the seventh pair ; 

 10, the fourth pair; 11, the fifth pair. 



Fig. 192. as the diameter of the pupil is regu- 



lated through the Optic nerve. In the 

 involuntary start, however, which is 

 occasioned by a loud and sudden sound, 

 we have an example of a consensual 

 movement excited through the Audi- 

 tory nerve, which is evidently analogous 

 to the closure of the eyes to a strong 

 light. In certain morbidly impressible 

 states of the nervous system, as will be 

 shown hereafter, the effect of sounds on 

 the motor apparatus is far more re- 

 markable. It has been attempted by 

 Flourens to show that the division of 

 the Auditory nerve, which proceeds to 

 the Semicircular canals, has functions 

 altogether different from that portion 

 which supplies the Vestibule and 

 Cochlea. This inference, however, is 

 grounded only upon the movements 

 exhibited by animals in which these 



nerves are irritated ; which movements are capable of a different explanation 



( 736). 



744. The nerves which minister to the sense of Taste are destitute of the 

 peculiarities which distinguish the preceding; being no other than certain 

 branches of ordinary afferent nerves the Fifth Pair and Glosso-pharyngeal 

 ( 717) the peculiar endowments of which seem to depend rather upon the 

 structure and actions of the papillae at their peripheral extremities than upon 

 anything special in their own characters ; for, as in the case of the ordinary 

 nerves of " common" sensation, mechanical irritation applied to them calls forth 

 indications of pain. From the observations and experiments of M. Cl. Ber- 

 nard, 1 it appears that the Facial nerve (portio dura of the 7th) supplies some 

 condition requisite for the sense of Taste, through the branch known as the 

 Chorda Tympani, which is the motor nerve of the Lingualis muscle. When 

 paralysis of the Facial exists in Man, the sense of taste is very much impaired 

 on the corresponding side of the tongue, provided the cause of the paralysis be 

 seated above the origin of the Chorda Tympani from its trunk. Similar results 

 have been obtained from experiments upon other animals. The nature of the 

 influence afforded by this nerve is entirely unknown ; and it is the more obscure, 

 as the Chorda Tympani contains no sensory filaments. 



745. To the sense of Touch, all the afferent nerves of the body (save the 

 nerves of special sense) appear to minister; in virtue according to the doctrine 

 already propounded of the direct connection of certain of their fibrils with the 

 Sensorium commune. But the degree in which they are capable of producing 

 Sensations does not bear any constant relation to their power of exciting reflex 

 actions. Thus, the Glosso-pharyngeal is not nearly so sensitive as the Fifth 

 pair ; though more powerful as an excitor nerve. The Par Vagum appears to 

 have even less power of arousing sensory changes ; although it is the most 

 important of all the exciters to reflex action. So, again, the afferent nerves of 

 the inferior extremities, in Man, are less concerned in ministering to sensations, 

 than are those of the superior ; and yet they appear to be much more efficient 

 as exciters to muscular action. These differences may be accounted for, by 

 supposing that the proportion which the fibres, having their centre in the 



Archives Generates de Medecine," 1844. 



