166 RICHARD ASSHETON, M.A. 



development of the eye." On p. 85 Professor Sch'afer gives what 

 appears to be an abstract from 0. Hertwig's ' Lehrbuch ' (9), pp. 402 

 — 406, in which Hertwig gives His's more recent opinions on the 

 subject. 



His (10) originally considered it probable that the nerve-fibres 

 arose within the brain and travelled towards the retina, but recently 

 (12) has changed his views, and considers it more likely that they 

 arise from neuroblasts within the retina and grow centralwards. 



Thus it seems that the tendency of writers of this country is to 

 adhere to the older view in spite of strong evidence in favour of the 

 newer advanced by foreign authors. 



Moreover, if we accept the theory that all nerve-fibres are out- 

 growths from nerve-cells, we have the advantage of knowledge of a 

 peculiarly fascinating nature, and the comprehension of the structure 

 and development of the central nervous system is rendered clearer to 

 both teacher and pupil. 



The beautiful works of His, Cajal, and others — but more especially 

 of His — having once taught us what to look for, it is an easy matter 

 with ordinary care in the preparation of specimens and with thin 

 sections, to find the early commencement of nerve-fibre tracts, arising 

 as they do from definite groups of neuroblasts. I do not doubt that 

 the conclusions of His are correct, namely, that the fibres of the dorsal 

 roots of the spinal nerves and the sensory fibres of the cranial nerves 

 arise as processes from neuroblasts of the spinal ganglia in the one 

 case, of the cranial ganglia in the other case, and grow inwards to the 

 central nervous system. 



Similarly also the sensory fibres of the sense-organs may be expected 

 to grow inwards from the sensory epithelium of the sense-organ to the 

 central nervous system. 



In no sense-organ can the outgrowths of the fibres to the central 

 nervous system be more easily traced than in the case of the fibres 

 arising in connection with the optic organ ; as the distance between 

 the place of origin of the fibres (in the retina) and the final destination 

 of the fibres (the brain) is relatively greater than in such cases as the 

 distance between the olfactory epithelium and brain, or between 

 ganglia of cranial and spinal nerves and the neural tube, in which 

 cases it is exceedingly difficult to trace the centralwards growth of the 



