BRAIN AND NERVOUS SYSTEM. 



329 



perform separate offices ; each effects its own purpose, and no 

 more; and each one neither interferes with, nor can take the 

 place of, the other. If, from any cause, the nerve of motion 

 alone is disordered, ^separated, or pressed, the muscles cannot 

 move, but the power of feeling remains. But if the other 

 nerve that of sensibility is injured, there is no feeling, but. 

 a power of motion. If the motory nerve that connects the 

 brain with the muscles of the jaw and lips is divided in any 

 animal, he may still smell his food with his nostrils, and feel it 

 with his lips, but he cannot open his mouth to take it in and 

 masticate it. But if the sensory nerve is divided, the animal 

 cannot feel the food with his lips, although he can move them 

 to take it in. 



754. These nerves have separate roots in the spinal cord; 



FIG. LXV. 



Section of the Spinal Cord. 



a 



the sensory nerve arises in the 



posterior, (Fig. LXV.~d,r/,c?,rf,) 



and the motory nerve in the an- 



terior part.- (e,e,e,e.) These 



nerves are united (c, c, c, c,) 



soon after they leave the spine, 



and a"re for some distance in- 



cluded in the same sheath, yet 



their branches are not equally 



distributed to all the parts of c 



the system. The skin has no 



power of motion, but it has a, b. Section of the cord. 



great sensibility. It is, there- c, c,c,c. Spinal nerves. 



~ <! d.d,d,d. Posterior or sensory 



fore, very largely supplied with roots of the spin>al nerves> 



sensory nerves, but not with e, e, e, e. Anterior or motory roots 

 motory nerves. On^the other of the s P inal nerves - 

 hand, the muscles, being exclusively organs of motion, are 

 very largely endowed with motory nerves, but with a limited 

 supply of the sensory fibres. In consequence of this, when 

 the surgeon amputates a limb, great pain is suffered while the 

 knife is cutting through the skin, and comparatively little 

 when the muscles are divided. 

 28* 



