100 PHYSIOLOGY. 



greater in elasticity than in density ,- certain stimulants, parti- 

 cularly the repetition of them, may operate in this way ; and I 

 think that to this head we may refer the power of tension in 

 giving irritability. It has been the observation of every physi- 

 cian, that plethoric systems, in which there is the greatest ten- 

 sion, are always the most irritable ; so that persons who are dis- 

 posed to plethora, viz. such as are of the sanguine temperament, 

 the female sex, and indeed both sexes about the time of their 

 acme, which is their most plethoric state, have the most irritable 

 systems ; however the theory of this is certainly difficult. I have 

 said that tension gives a certain degree of vigour, and hitherto I 

 have considered it in that view ; vigour is very often opposed to 

 increased mobility or irritability ; if, therefore, they here co-exist, 

 there must be some singular combination of circumstances. But 

 I conclude that the fact is sufficiently certain, that plethoric 

 habits are the most irritable, or that irritability is most evi- 

 dent where plethora is present, or in systems that are liable to 

 haemorrhagy ; and, therefore, I have sometimes said that hae- 

 morrhagic systems are the most moveable, so that the female 

 sex, who are most liable to haemorrhagy, are also more irritable, es- 

 pecially at the time of menstruation, when the most remarkable 

 symptoms of mobility occur in the female sex ; and from the 

 same effects of a plethoric habit we have a clear explanation of 

 the cause mentioned by Gaubius, the ' genus vitae lautum sine 

 exercitatione, 1 which is the most effectual in giving a plethoric 

 state, and therefore in giving a remarkably irritable system. 



" The fourth cause of irritability we find in the effects of ha- 

 bit. This operates in two ways, 1. in consequence of the law 

 mentioned in CXIV. which is, that the repetition of motion 

 gives greater facility of motion, i. e. irritability. A matter of 

 more curiosity which I have to add, is, that repeated motions often 

 put on the appearance of spontaneity, when they are associated 

 with sensations or motions, which, by the laws of our systenij 

 periodically return. In that way I particularly explain the daily 

 revolutions in our system, and how various other circumstances 

 will continue to recur, in consequence of an association with 

 other things that do occur in that circle ; and, with regard to 

 Pathology, how several diseases > which depend upon a state of 

 irritability, as epileptic, hysteric, asthmatic affections, become 



