60 LETTERS TO BROTHER JOHN. 



of the parent artery flow into the branch which 

 proceeds from it. The larger veins, also, are formed 

 by the absolute union of smaller ones ; so that the 

 contents of the smaller flow into and mingle with 

 the contents of the larger : but between the large 

 nerves and the branches which proceed from them 

 there is no union nor communication whatever ; 

 they are merely in juxta-position a bundle of se- 

 parate threads, bound up together, and inclosed in 

 one common sheath. When, therefore, a nerve 

 gives off a branch, that branch merely parts com- 

 pany, to travel along another road. Every nerve 

 therefore, however minute, is a distinct thread ; 

 having one of its extremities fixed in the brain or 

 spinal marrow, and the other in that point of the 

 body on which it terminates. If it were not for 

 this peculiar arrangement, all our different sensa- 

 tions would be jumbled into one. If we touched a 

 round body with one hand, and a square one with 

 the other, before the two impressions reached the 

 brain they would become mingled ; so that the idea 

 which we should derive from these two impressions 

 would be a sort of hybrid idea of a something 

 neither round nor square. 



There is one pair of nerves, included by me 

 among those arising from the brain, which pos- 



