THE DISCOVERY OP MOTOR AND SENSORY NERVE CHANNELS. 295 



the words ' obvious loss of motion ' to the words ' obvious loss of 

 sensation,' but makes no mention of Mayo.) 



In the second number of his ' Commentaries ' (1823) at p. 9 

 Mayo is quite clear that the fifth consists of two portions, sensory and 

 motor, and is led from the analogy between the fifth and the spinal 

 nerves 



' to conjecture that the double roots of the spinal nerves have functions corre- 

 sponding with those of the fifth, and that the large posterior portion of each 

 spinal nerve, with its ganglion, belongs to cutaneous sensation, and the anterior 

 branch to voluntary motion. When I was engaged in experiments to determire 

 the fact, M. Magendie's were published, which established the justness of my 

 conjecture.' 



Thus Mayo in 1823 and Bell in 1824 were clear about motor and 

 sensory nerves in general, and about the fifth nerves in particular. 

 But in 1821, i.e., before Magendie's publication of 1822, neither Bell 

 nor Mayo possessed any clear view as to the motor and sensory func- 

 tions of the fifth or of any other mixed nerve. 



Bell said at that time '(' Phil. Trans. R.S.,' 1821, p. 404): — 



'The nerves of the spine, the tenth or sub-occipital nerve, and the fifth or 

 Trigeminus of the system of Willis, constitute this original and symmetrical 

 system. All these nerves agree in these essential circumstances; they have 

 all double origins; they have all ganglia on one of their roots; they go out 

 laterally to certain divisions of the body; they do not interfere to unite the 

 divisions of the frame ; they are all muscular nerves ; ordering the voluntary 

 motions of the frame; they are all exquisitely sensible; and the source of the 

 common sensibility of the surfaces of the body : when accurately represented on 

 paper, they are seen to pervade every part; no part is without them; and yet 

 they are symmetrical and simple as the nerves of the lower animals. 



If the nerves be exposed in a living animal, those of this class exhibit the 

 highest degree of sensibility; while on the contrary, nerves not of this original 

 class or system are comparatively so little sensible as to be immediately dis- 

 tinguished, insomuch that the quiescence of the animal suggests a doubt 

 whether they be sensible in any degree whatever. If the fifth nerve, and the 

 portio dura of the seventh, be both exposed on the face of a living animal, there 

 will not remain the slightest doubt in the mind of the experimenter which 

 bestows sensibility. If the nerve of this original class be divided, it in no 

 measure deprives the parts of their sensibility to external impression. 



At our present level of knowledge — 1911 — or even at the level of 

 1830 it is possible to read into this passage that Bell meant that the 

 fifth is sensory and the seventh motor. But from the remainder of the 

 paper it soon becomes evident to us that this was not his meaning in 

 1821, and that he was contrasting the sensibility of an original or 

 regular nerve of animal life, i.e., the fifth, with the insensibility of 

 other nerves, e.g., his respiratory system of nerves. He does not 

 say that the portio dura is motor in contrast to the fifth sensory, but 

 only that the portio dura is insensitive in comparison with the fifth, 

 which is very sensitive. The distinction in his mind is similar to 

 that by which he had been misled in 1810 in thinking about the nerve- 

 roots, when he says: — 



' It is almost superfluous to say that the part of the spinal marrow having 

 sensibility comes from the eerebrvm; the posterior and insensitive part of the 

 spinal marrow belongs to the cerebellum. Taking these facts as they stand, is 

 it not most curious that there should be thus established a distinction in the 

 parts of a nerve, and that a nerve should be insensitive.' — ' Letters,' p. 171. 



