INTRODUCTION. 37 



more activity and force, in proportion as their ex- 

 citement, that is, the nervous fluid, is more abun- 

 dant ; and as this fluid is itself produced by a par- 

 ticular secretion, its abundance must depend upon 

 that of the medullary substance which secretes it, 

 and the accessions which this medullary substance 

 receives from the blood. 



In animals possessed of a circulating system, the 

 blood travels to its destination through the arteries, 

 which transport it along by means of their irritabi- 

 lity and that of the heart. If these arteries are 

 strongly irritated, they act with greater yigour, and 

 propel a greater quantity of blood. The nervous 

 fluid then becomes more abundant, augmenting the 

 local sensibility, and increasing in its turn the arte- 

 rial irritability*. This mutual action may be some- 

 times carried to an excessive pitch. Its ordinary 

 appellation is orgasm, but when it becomes painful 

 and permanent it is termed inflammation. The 

 commencement of this irritating action may some- 

 times take place in the nerve itself, when it is sub- 

 jected to the influence of lively sensations. 



Cuvier, but atthe same time restricts its cause to those nerves 

 •which are given off from ganglia — which accompany the blood- 

 vessels and supply their coats, and which are removed from the 

 influence of the will, owing to their being derived from different 

 centres from that which gives rise to sensation and volition. 



* "The nervous fluid becomes more abundant;" the arterial capil- 

 laries are consequently excited, and this excitation having extended 

 to these capillaries which supply the ramifications of voluntary 

 nerves, local sensibility is thereby augmented, and thus the arte- 

 rial irritability is still farther increased. 



