Manchester Memoirs, Vol. xlii. (1898), No. \%, 3 



of, taking place in the mechanism during the development 

 of the psychical effect ? 



The mechanism may be briefly described as a chain 

 of linked units, the unit being the nerve cell, or, as it is 

 sometimes called, the netirone. In order to make clear 

 the characteristic features and properties of the neurone, 

 I must for a while pass away from the special visual 

 mechanism to the general nervous system. 



A nerve cell consists, like the cell or unit of any other 

 of the tissues of the body, of a nucleus lying in the midst 

 of a mass of material spoken of as the cell body. In 

 the typical nerve cell the cell body is prolonged into a 

 number of processes, it may be many, it may be few, 

 stretching away from the neighbourhood of the nucleus. 

 The majority of these processes, all in fact but one, 

 branching rapidly and extensively in ramifications or 

 arborisations, end in fine points at no very great distance 

 from the nucleus ; these processes are called dendrites oxden- 

 drons. One process does not ramify in this way ; it pursues 

 an undivided course, it may be for a very long distance from 

 the nucleus. Moreover, the material of which it is com- 

 posed differs in microscopic appearance and in its reac- 

 tions towards colouring and other chemical reagents from 

 the material of the dendrites. Further, unlike, at least, 

 the typical dendrite, it usually developes at a greater or less 

 distance from the nucleus, around the material which is 

 the prolongation of the substance of the cell body, a 

 coating of a peculiar, largely fatty nature, spoken of as 

 myelin or medulla. This undivided process is called the 

 axis-cyli?ider process^ or neuraxon^ or, more briefly, axo?i ; 

 it has also been called the neurite. Though not ramifying 

 like the typical dendrites, the axon not unfrequently, indeed 

 usually, sends off at intervals, generally at right angles to 

 itself, fine lateral twigs resembling itself, but more 

 delicate, called collaterals, and, after pursuing its 



