NERVOUS FIBRES. 149 



white substance is less consistent in the foetus, it separates the 

 more readily, and the artificial generation of such globules is 

 very easy of observation in foetal nerves. 



The growth of nerves neither proceeds from the circum- 

 ference towards the central organs, nor vice versd, but their 

 primary cells are included amongst those from which every 

 organ is formed, and which, so far at least as their appearance 

 is concerned, present no marks by which they can be distin- 

 guished from other cells. They arc first characterized as 

 nerves, when they become arranged in rows and coalesce to 

 form a secondary cell. After that coalescence each nervous 

 fibre forms a separate cell, which pursues an uninterrupted 

 course from the organ, in which its peripheral extremity is 

 situated, to the central organ of the nervous system. The 

 white substance of nerves does not appear to be formed at 

 so early a period in their peripheral extremities, as it is in 

 their trunks. The Medizinischen Zeitung for August 1837, 

 contains a description which I gave of some nerves from the 

 tail of frog's larvae, which presented an appearance quite dif- 

 ferent from ordinary nerves, inasmuch as they had a pale con- 

 tour and no perceptible cavity. They were nerves in an early 

 stage, previous to the development of the white substance. 

 They represent the only form of nervous matter which we find 

 in the tail of very young larvae. Some isolated nerves, having 

 the ordinary appearance of the dark contours, gradually make 

 their appearance, and afterwards increase in quantity j they 

 were first observed in the neighbourhood of the muscular fas- 

 ciculus which traverses the middle of the tail. The development 

 of the white substance appears therefore to advance from the 

 trunks towards the circumference. These white fibres become 

 more minute and paler towards the periphery. Sometimes such 

 a fibre seems to terminate suddenly with even an incomplete 

 acuniination. But, on a more accurate observation, some ex- 

 tremely delicate, very thin filaments are generally seen going 

 off from it. The pale immature fibres in the tail of the 

 frog's larvae also subdivide. A question now arises arc those 

 more minute fibres (which at least present an appearance of 

 subdivision) already prepared within an ordinary white primi- 

 tive nervous fibre, or are they actual subdivisions? Since each 

 nervous fibre is a secondary cell, and retains its character as 



