LETTERS TO BROTHER JOHN. 59 



through a round hole in the back part of the scull. 

 So great is its resemblance to a tail, that it has been 

 called cauda cerebri ; that is, the toil of the brain. 



From the brain and spinal marrow there arise 

 forty-three pairs of nerves ; twelve from the brain, 

 and thirty-one from the spinal marrow. The 

 nerves are whitish cords; and every large nerve 

 consists of a bundle of small ones; and these small 

 ones consist of bundles of still smaller, as a skein 

 of thread consists of a number of single threads, 

 and as every single thread consists of a number of 

 still smaller threads, viz. the fibres of the cotton. 

 As a large nerve proceeds from its origin to its 

 termination, every now and then one or more of 

 the threads, of which it is composed, parts company, 

 and takes a course of its own. As these proceed, 

 one or more of the strands, of which they also are 

 composed, disjoins itself from the fellowship of the 

 others, and takes a course of its own ; and so on, un- 

 til the whole have been separated into microscopic 

 filaments of undistinguishable minuteness. You 

 will observe here a remarkable difference in the 

 manner in which nerves are distributed from that 

 in which arteries are given off. The branch of an 

 artery arises directly/Vwn that artery : there is a 

 communication between them : so that the contents 



