SUPPLY OF BLOOD TO NERVOUS TISSUE. 



215 



that no manifestation of nervous power can take place, unless this change can 

 be effected. There is strong reason to believe, further, that this change es- 

 sentially consists in the union of oxygen conveyed by the arterial blood, with 

 the elements of the proper nervous matter ; and that this union consequently 

 involves the death and disintegration of a certain amount of the nervous tis- 

 sue, the reproduction of which will be requisite, in order that the systsm 

 may be maintained in a state fit for action. This reproduction is effected by 

 the nutritive process, which takes place at the expense of other constituents 

 of the blood; and it will proceed most vigorously in the intervals, when the 

 active powers of the nervous system are not being called into operation ( 292 

 296). 



251. The proofs of this continual waste and reproduction of the Nervous 

 substance, will be partly found in the appearance of the products of its de- 

 composition in the excretions, and in the demand which is set up for the ma- 

 terials for its reparation ; these being found to accord in amount, as will be 

 shown hereafter, with the degree of its functional activity. But evidence of 

 another kind may be drawn from the microscopic appearances observable in 

 the cortical substance of the Brain. It seems probable, from the observations 

 of Henle, that there is as continual a succession of nerve-cells, as there is of 

 epidermic cells; their development commencing at the surface, where they 

 are most copiously supplied with blood-vessels from the pia mater; and pro- 

 ceeding as they are carried towards the inner layers, where they come into 

 more immediate relation with the tubular portion of the nervous tissue. This 

 change of place is probably due- to the continual death and disintegration of 

 the mature cells, where they are connected with the fibres, and the equally 

 rapid production of new generations at the external surface ; the newly- 

 formed epidermic cells being thus carried inwards, in precisely the same man- 

 ner that the epidermic cells are carried outwards. 



252. The first development of the Nerve-tubes appears to take place, likg 

 that of Muscular fibre, by the coalescence of a number of primary cells into 

 a continuous tube ; for although the primary nervous cell has not yet been 

 made out with precision, the nuclei of what seem to be the original cells may 

 frequently be seen in the fullv- 



formed tube, lying between their 

 membranous walls, and the white 

 substance of Schwann (111, c). 

 When first a nerve-fibre can be re- 

 cognized as such, it has a strong re- 

 semblance to the gelatinous fibres 

 of the sympathetic trunks ; being a 

 cord of small diameter, without any 

 clear distinction between the tube 

 and its contents, of granular consist- 

 ence, and havingf nuclei at no great 

 distance from each other. The 

 substance of the fibre, at this pe- 

 riod, seems to correspond with the 

 axis-cylinder of the fully-formed 

 nerve-tube ; the white substance of 

 Schwann is subsequently deposited 

 around it, separating it from the 

 membranous 



[Fig. 123. 



Various stages of the development of nerve ; a. 

 Earliest stage, b. Detached fibre, c. Nucleated fibre 

 in the lower part of which, d, the white substance of 

 Schwann has begun to be deposited, e. Nucleus in a 

 more fully-formed fibre between the white substance 

 and tubular membrane. /.Displays the tubular mem- 

 brane, the contained matter having given way. (After 

 Schwann.)] 



tubular envelope. 

 The first development of the vesi- 

 cular substance appears to take place on the same plan with its subsequent 

 renewal. 



