254 THE VITAL FUNCTIONS. 



pcriments have shown that the due performance of the vital 

 functions of digestion, of circulation, and of secretion, re- 

 quires the presence of an ai^ency, derived from different 

 parts of tlie hrain and spinal marrow, and rci^ulatino; the or- 

 der and combinations of the actions of the organs which are 

 to perform those functions. The same influence, for exam- 

 ple, which increases the power of secretion in any particu- 

 lar gland, is found to increase, at the same time, the action 

 of those blood vessels which supply that gland with the ma- 

 terials for secretion; and conversely, the increased action of 

 the blood vessels is accompanied by an increased activity 

 of the secreting organ. Experience also shows that when 

 the influence of the brain and spinal marrow is intercepted, 

 although the afflux of blood may, for a time, continue, yet 

 the secretion ceases, and all the functions dependent upon 

 secretion, such as digestion, cease likewise. Thus, the ner- 

 vous power combines together different operations, adjusts 

 their respective degrees, and regulates their succession, so as 

 to ensure that perfect harmony which is essential to the at- 

 tainment of the objects of the vital functions; and thus, not 

 only the muscular power which resides in the vital organs, 

 but also the organic affinities which produce secretion, and 

 all those unknown causes which effect the nutrition, deve- 

 lopment, and growth of each part, are placed under the con- 

 trol of the nervous povver.^ 



Although we are entirely ignorant of the nature of the 

 nervous power, we know that, when employed in the vital 

 functions, it acts through the medium of a particular set of 

 fibres, which form part of the nervous system, and are classed, 

 therefore, among the nerves. The principal fflaments of this 

 class of nerves compose what is called the sympathetic 

 nerve, from its being regarded as the medium of extensive 



* As the functions of plants arc sufficiently simple to admit of being con- 

 ducted witliout the aid of muscidar power, still less do tiiey require the as- 

 sistance of tlie ncn'ous encrg-y: both of which properties are the peculiar at- 

 tributes of animal vitality. We accordingly find no traces citlier of nervous 

 or of musctdar fibres in any of the vegetable structures. 



