THE NERVOUS SYSTEM. 157 



the anterior and posterior roots unite to form the trunk 

 of a nerve, which is distributed, by means of branches, 

 to the various organs of that part of the body which this 

 nerve is designed to serve. The spinal nerves supply 

 chiefly the muscles of the trunk and limbs and the ex- 

 ternal surface of the body. 



15. The tissue composing the nerves is entirely of the 

 white variety, or, in other words, the nerve-fibres ; the 

 same as we have observed forming a part of the brain. 

 But the nerves, instead of being soft and pulpy, as in the 

 case of the brain, are dense in structure, being hardened 

 and strengthened by means of a fibrous tissue which sur- 

 rounds each of these delicate fibres, and binds them to- 

 gether in glistening, silvery bundles. Delicate and mi- 

 nutely fine as are these nerve-fibres, it is probable that 

 each of them pursues an unbroken, isolated course, from its 

 origin, in the brain or elsewhere, to that particular point 

 which it is intended to serve. For, although their extrem- 

 ities are often only a hair's breadth distant from each other, 

 the impression which any one of them communicates is 

 perfectly distinct, and is referred to the exact point whence 

 it came. 



16. This may be illustrated in a simple manner, thus: 

 if two fingers be pressed closely together, and the point of a 

 pin be carried lightly across from one to the other, the eyes 

 may be closed, and yet we can easily note the precise instant 

 when the pin passes from one finger to the other. If the 

 nerve-fibres were less independent, and if it were necessary 

 that they should blend with and support each other, all 

 accuracy of perception would be lost, and all information 

 thus afforded would be pointless and confused. These sil- 

 very threads must, therefore, be spun out with an infinite 

 degree of nicety. Imagine, for instance, the fibre which 



1 5. The nerve tissue ? Its character ? Course of each nerve fibre ? 



1 6. How may we illustrate the fact ? The fibre connecting the brain with a 

 point in the foot ? 



