140 THE HUMAN BODY. 



extremity. The man who has lost an arm or a leg, feels 

 pains, which he refers not to the stump which remains, but 

 to the hand or the foot which he has lost. It is the nervous 

 filaments primitively destined to these parts which are the 

 seat of the pain, and which now transmit it as coming from 

 the organ to which they formerly gave sensibility. The same 

 effect is produced when a piece of skin has been transplanted 

 from the forehead to the nose by autoplasty; if the patient 

 be touched on the nose he feels the impression on his fore- 

 head. 



We shall see in treating of the senses, that tactile impres- 

 sions may be distinct on the tips of the fingers, at a distance 

 of one-fiftieth of an inch from each other; which implies that 

 there are two filaments at this interval from each other, running 

 directly to the brain, but we err if we reckon in this way the 

 extent of nerve subdivision, for every point in the skin, how- 

 ever small, is sensitive to the touch. It is by innumerable 

 ramifications, each of which contains at least one nervous 

 filament, that the nerves terminate in the organs, those of 

 motion to excite muscular contraction, and those of sensa- 

 tion to receive and transmit impressions. 



Functions of the cranial nerves. Like those which spring 

 from the spinal cord, the nerves of the brain are divisible 

 into motor and sensory nerves. Among these last some are 

 endowed with a special sensibility, as the olfactory, the optic, 

 and auditory nerves; the others transmit general sensations. 

 Several of the cranial nerves are made up of filaments of 

 different orders, and are formed by the union of nerves of 

 general sensation, of special sensation, or of motion. Like 

 the spinal nerves after the union of their roots, they form 

 cords, mixed in their functions as a whole, but distinct in 

 those of their several filaments. The analogy between the 

 cranial and the spinal nerves is completed by the branches 

 which go from the cranial nerves of sensation to the great 

 sympathetic, and by the gray fibres which are seen near the 

 origin of the cranial nerves, and also near the posterior roots 

 of the spinal nerves. In the cranium the motor nerves 

 emerge from the prolongation of the anterior fascicles of 

 the cord in which the spinal motor nerves originate, 



