REPORT ON THE PHYSIOLOGY OF THE NERVOUS SYSTEM. 87 



the thirty-one pau's of spinal nerves, becomes, after the union 

 of their roots, invested with a twofold endowment, and conti- 

 nues so throughout their entire course and final distribution to 

 the muscular tissue. It would appear, indeed, from a later 

 paper of Sir Charles Bell*, that nerves of sensation, as well as 

 of motion, are necessary to the perfect action of the voluntary 

 muscles. "Between the brain and the muscles there is a circle 

 of nerves ; one nerve conveys the influence from the brain to 

 the muscle, another gives the sense of the condition of the 

 muscle to the brain." In the case of the spinal nerves this 

 circle of intercourse is at least probable ; but proof of its ne- 

 cessity must be obtained, from observing the habitudes of those 

 encephalic nerves, which minister exclusively to motion. Now 

 it is found, on minute dissection, that the muscles of the eye- 

 ball, which are supplied by the third, fourth and sixth motive 

 nerves, also receive sensitive filaments from the ophthalmic 

 branch of the fifth ; and that the muscles of the face, to which 

 the portio dura is distributed, are also furnished with branches 

 of sensation from the fifth. Sir Charles Bell has further shown 

 that the muscles of the lower jaw, to which the motive im- 

 pression is propagated by the muscular branch of the inferior 

 maxillary, draw nervous supplies also from the ganglionic or 

 sensitive branch of that division of the fifth pair. This com- 

 plicated provision has its origin, he supposes, in its being "ne- 

 cessary to the governance of the muscular frame that there 

 should be consciousness of the state or degree of action of the 

 muscles." 



III. The olfactory, auditory and optic nerves are gifted with 

 a special sensibility to the objects of the external senses, to 

 which they respectively minister. Magendie seems to have 

 been the first to prove, experimentally, that they do not also 

 share the common or tactile sensibility. He exposed the olfac- 

 tory nerves, and found that, like the hemispheres of the brain 

 from which they spring, they are insensible to pressure, prick- 

 ing, or even laceration. Strong ammonia was dropped upon 

 them without eliciting any signs of feeling. The optic nerve, 

 and its expansion on the retina, participate with the olfactory 

 in this insensibility to stimulants. This was proved by Ma- 

 gendie in the human subject as well as in animals. In perform- 

 ing the operation of depressing the opaque lens, he repeatedly 

 touched the retina in two different individuals without awaken- 

 ing the slightest sensation. The portio mollis, or acoustic nerve, 

 was also touched, pressed, and even torn without causing pain, 



* Philosophical Transactions, 1S2G, p. 163. 



