i8z THE POPULAR SCIENCE MONTHLY. 



the spinal nerves) by a double root, to be the nerve of ordinary (or 

 voluntary) motion for the muscles of the face generally, as well as of 

 sensation for its sensory surfaces. The analogy of the fifth pair to the 

 spinal nerves (which was no new idea) seemed to him to be further 

 indicated by the existence of a " ganglion " upon its larger root, corre- 

 sponding with that which is seen on the posterior roots of the spinal 

 nerves. Following up this train of reasoning, he instituted experi- 

 ments with the view of determining what function the fifth pair had 

 in virtue of its double root, which the seventh pair had not. And as 

 he found that division of the seventh pair, while partially paralyzing 

 the muscles of the face, did not in any perceptible degree impair its 

 sensibility, while section of either of the three divisions of the fifth 

 pair destroys the sensibility of the part of the face it supplies, he came 

 to the conclusion that the sensory endowments of the fifth pair are due 

 to its possession of a double root ; a conclusion which he strengthened 

 by the consideration that the third, fourth, and sixth nerves which, 

 being distributed exclusively to the muscles of the eyeball, can not be 

 supposed to have any but motor endowments all arise by single roots. 



In this way, Bell was led to assign to the two roots of the spinal 

 nerves the same double function which he attributed to the two roots 

 of the fifth pair of nerves of the head ; and thence to assign the sen- 

 sory function to the posterior roots, because, like the second root of 

 the fifth, they bore ganglia before uniting with the motor roots.* Now, 

 to say that Bell, by this train of reasoning, discovered the motor and 

 sensory functions of the anterior and posterior roots of the spinal 

 nerves, is utterly preposterous. He had not even truly determined (as 

 the event proved) the true functions of the fifth and seventh nerves of 

 the head. And the extension of his conclusions regarding the double 

 roots of the fifth pair, to the spinal nerves generally, had rather the 

 character of a happy guess than of a logical sequence. No scientific 

 physiologist at the present time would think himself justified in put- 

 ting forward such an extension as more than a suggestion, to be con- 

 firmed or negatived by experimental evidence. And let it not be for- 

 gotten, moreover, that it was experiment alone which afforded Bell 

 any reason whatever for attributing a sensory function to the gangli- 

 ated root of the fifth pair ; and that, without this basis, the question 

 of the spinal nerves remained exactly in the condition in which he had 

 taken it up. 



It is, indeed, not a little curious that in the two memoirs (1821 

 and 1822) in which Bell presented to the Royal Society the results of 



* It is a significant indication of the chaotic ignorance which prevailed on this subject 

 " sixty years since," that, as Bell himself informs us, he found himself met, when first 

 groping at the notion of the sensory endowments of the posterior roots of the spinal 

 nerves, by the current doctrine that the function of the ganglia is " to cut off sensation," 

 i. e., to allow these nerves to minister to the " vital and involuntary motions," without our 

 being made conscious either of those movements or of the impressions which excite them. 



