474 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



check or hindrance is interposed, by external influences, to that regular 

 sequence of changes, on which the continuance of its powers depends. 

 In this survey it will have been perceived, that the essential parts of 

 these operations are, in Animals as in Plants, completely independent 

 of the influence of that which constitutes the peculiar endowment of 

 Animals ; namely, the Nervous System. 



a. The Reduction of the food in the Stomach, by the solvent power 

 of the gastric fluid, is a purely chemical operation, with which the Ner- 

 vous System has nothing whatever to do, excepting that it perhaps 

 accelerates the process, by stimulating the Muscular coat of the stomach 

 to that peculiar series of contractions, which keeps the contents of the 

 cavity in continual movement, and favours the action of the solvent 

 upon it. 



b. In the process of Absorption, by which the nutritive materials, 

 with other substances, are introduced into the vessels, the Nervous Sys- 

 tem has no participation ; this being a purely vegetative operation, partly 

 dependent upon the simple physical conditions which produce Endos- 

 mose, and partly on a process of cell-growth. 



c. The Assimilation of the new material, effected, as we have seen 

 reason to believe, by another set of independent cells, can receive but 

 little influence from the Nervous System, and is obviously capable of 

 taking place without its aid. 



d. The Circulation of the Blood, again, though dependent in part 

 upon the impulsive power of a Muscular organ, the Heart, is not on that 

 account brought into closer dependence upon the Nervous System ; for 

 we have seen that the contractions of the heart result from its own 

 inherent powers, so as to continue after it has been completely detached 

 from the body ; and that the capillary power, which is the chief agent 

 in the movement of the blood in the lower animals, and which exerts an 

 important subsidiary action in the higher, is the result of the exercise 

 of certain affinities between the blood and the surrounding tissues, in 

 which the Nervous System can have no immediate concern. 



e. The act of Nutrition, in which every tissue draws from the circu- 

 lating blood the materials for its own continued growth and develop- 

 ment, and by which it incorporates these with its own substance, is but 

 a continuance of the same kind of operation, as that which takes place 

 in the early development of the embryo, long anteriorly to the first 

 appearance of the nervous system, namely, a process of cell-develop- 

 ment and metamorphosis, which must be, from its very nature, indepen- 

 dent of Nervous agency. 



/. The same may be said of the Secreting operation in general; for 

 this essentially consists in the separation of certain products from the 

 blood, by cells situated upon free surfaces ; which thus remove those 

 products from the interior of the fabric. 



g. And the interchange of oxygen and carbonic acid, which takes 

 place between the atmosphere and the venous blood, when brought into 

 mutual relation in the lungs, and which is the essential part of the func- 

 tion of Respiration, is an operation of a merely physical character, with 

 which the Nervous system can have no direct concern. 



h. Finally, the development of the " sperm-germ-cells" in the one 



