58 LETTERS TO BROTHER JOHN. 



structure of the body (except the nails, hair, 

 enamel of the teeth, and scarf-skin), are composed 

 of arteries, veins, nerves, and absorbents. It is 

 true, that in the actual formation of the gland there 

 is only an artery concerned ; but this artery could 

 not produce the required secretion without the aid 

 of a nerve : it is, therefore, accompanied by a 

 nerve ; but this artery and nerve, like the rest of 

 the body, are constantly undergoing decay and re- 

 paration; and these cannot be effected without the 

 presence of lymphatics and arteries. There are, 

 accordingly, lymphatics arising from the gland, 

 which are constantly engaged in absorbing it ; and 

 there are arteries as constantly engaged in repro- 

 ducing what the lymphatics have removed : and 

 there are veins also, whose office it is to carry back 

 to the heart whatever portion of blood has not been 

 consumed in the work of reproduction. 



I have now to speak of the fourth principal struc- 

 ture which enters into the composition of the ulti- 

 mate tissue of the body I mean, the nerves. The 

 brain accurately fills the cavity of the scull. With 

 its general appearance you are probably acquainted, 

 from having seen the brains of animals. 



The spinal marrow is a tail-like elongation of the 

 brain; which elongation passes out of the head, 



