STRUCTURE OF NERVOUS TISSUE. 345 



a certain change of composition, so that no manifestation of nervous power can 

 take place, unless this change can be effected ( 358 362). There is strong 

 reason to believe, further, that this change essentially 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 tissue, the reproduction of which, therefore, will be 

 requisite, in order that the system 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. The proofs of this continual waste and reproduction of 

 the Nervous substance, are partly afforded by the appearance of the products of 

 its decomposition in the excretions, and by the demand which is set up for the 

 materials 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 presented by the 

 cortical substance of the Brain. For ganglionic vesicles may be observed in all 

 stages of development ; and numerous bodies resembling free nuclei are seen in 

 the granular substance in which these are imbedded. It is surmised by Henle, 1 

 from the comparative aspects of the external and internal layers of the cortical 

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

 dermic cells ; their development commencing at the surface, where they are most 

 Copiously supplied with bloodvessels from the pia mater ; and proceeding as they 

 are carried towards the inner layers, where they come into more immediate rela- 

 tion with the tubular portion of the nervous tissue. This change of place is pro- 

 bably 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 G-anglionic cells being 

 thus carried inwards, in precisely the same manner that the Epidermic cells are 

 carried outwards ( 240). 



347. The maintenance of the integrity of the Nervous tissue, by the due per- 

 formance of the nutritive operations, is not less dependent than that of the Mus- 

 cular ( 313), upon the continuance of its functional activity; indeed, it will be 

 presently shown, that degenerative changes manifest themselves at a much ear- 

 lier period after the interruption of that activity, so as to indicate that the de- 

 pendence is yet closer. It has long been known that, when disease of the Optic 

 nerve or of its ganglionic centre occasions complete Amaurosis, the Retina comes 

 in time to lose its characteristic appearance, and the portion of the Nerve in 

 front of the seat of the disease presents an atrophied aspect. And it has been 

 shown by Valentin/ that, notwithstanding the general appearance of the eye 

 may be unchanged, the texture of the retina becomes completely altered; for 

 it is found to be composed of white cylindrical threads interlaced together, with- 

 out presenting any appearance of the ganglionic vesicles or of the tubular nerve- 

 fibres proper to the part; and the yellow spot of Soemmering becomes paler, and 

 is at last indistinguishable. But if a very slight degree of sensibility to light 

 remain (showing that some power of transmission still exists in the optic nerve), 

 these changes are much less decided. Further, when the sight is destroyed by 

 a disease or injury which prevents the light from passing through the pupil, the 

 whole eye becomes more or less atrophied; and the retina and optic nerve are 

 found after death (if the morbid condition have lasted sufficiently long) to have 

 lost their characteristic structure. It is probable that in the latter case, as in 

 that of Muscles long disused, a renewal of the normal nutrition, and a re-esta- 

 blishment of the structural and functional integrity of the tissues, takes place 



1 "Traite d'Anatomie Generate," traduit par Jourdan; torn. ii. p. 330. 



2 "De Functionibus Nervorum Cerebralium, et Nervi Sympathetic!," Bernse, 1839. 



