472 INFLUENCE OF THE NERVOUS SYSTEM 



the blood-vessels, accompanying them to their minutest ramifications ; and it 

 will be hereafter shown ( 732), that the fibrous (issue of the walls of the ar- 

 teries is probably susceptible of influence from these nerves. There can be 

 little doubt, therefore, that they constitute the channel through which Emo- 

 tions operate, in producing sudden distension of particular parts of the vascu- 

 lar system, as in blushing, erection, &c. And to the same kind of influence, 

 more gradually exerted, we may very probably attribute the regulation of the 

 supply of blood which passes to different secreting organs, in varying condi- 

 tions of the system. 



624. But the Sympathetic System does not consist of Cerebro-Spinal fila- 

 ments alone; nor is its influence exerted only upon the motor or contractile 

 tissues of the body. There is good evidence, that the Nervous System has 

 an immediate action upon the molecular changes, which constitute the func- 

 tions of Nutrition, Secretion, &c. ; and the channel of such influence is pro- 

 bably to be found in that system of peculiar fibres formerly described ( 244), 

 which constitutes a considerable proportion of the Visceral nerves, existing 

 much more sparingly in most of the Cerebro-Spinal, but being abundant in 

 the Fifth pair. There is no valid reason, however, to believe that any of the 

 processes of Nutrition and Secretion are dependent upon this, or any other, 

 kind of Nervous agency. These processes go on with great rapidity and 

 energy in the Vegetable kingdom, in which nothing approaching to a Nervous 

 System exists ; and in the Animal kingdom they take place with equal vigour, 

 long before the least vestige of it appears. The Embryological researches 

 of Dr. Barry have fully proved, that in the earliest condition of fffital life, the 

 germ consists but of a congeries of cells, which have all originated in a single 

 one ; and from this mass, the several tissues are gradually generated, by a 

 process which is technically called Mstolqgical* transformation, one set of 

 cells being converted into muscular tissue, anther into nervous tissue, another 

 into mucous membrane, and so on. Now since this is the case, it is evident 

 that all these processes of development must take glace, in virtue of the inhe- 

 rent properties of the primary tissue itself; since no nervous influence can be 

 supposed to operate, before nerves are called into existence. Throughout the 

 Animal body, it may be observed that, the more Vegetative the nature of any 

 function, the less is it connected with the Nervous System ; and all the ex- 

 periments, which have been regarded as proving that the Organic functions 

 are dependent upon Nervous influence, are really explicable, fully as well, 

 upon the supposition that they are capable of being affected by it, either in 

 the way of excitement or retardation (see 415). Moreover, there is abundant 

 evidence, that Secretion may take place after the death of the general system, 

 through the persistence of certain molecular changes, of which the essential 

 conditions are not immediately altered ; thus Mr. T. Bell mentions that, in 

 dissecting the poison apparatus of a Rattle-snake, which had been dead for 

 some hours, the poison continued to be secreted, so fast as to require being 

 occasionally dried off. This is precisely what might have been anticipated, 

 from the independent power of growth in the secreting cells ; and other acts 

 of Nutrition are recorded to have occurred under similar circumstances. In 

 such a case, the Animal body is reduced to the condition of a Plant; since 

 the influence of the Nervous system must then be entirely extinct. Upon 

 those Avho maintain that Nervous agency is a condition essential to those mo- 

 lecular actions, of which Nutrition and Secretion consist, it is incumbent, 

 therefore, to offer some more unexceptionable proof of their position than has 

 yet been given ; since their doctrine is opposed by so many considerations of 

 great weight. 



* This term is used in contradistinction to morphological ', which applies to the alterations 

 in the/orm of the several parts of the embryo. 



