154 PHYSIOLOGY. 



perhaps their operation may be that of their changing the nature 

 of our fluids, by their abstracting a considerable portion of the 

 acid which should have entered into the composition of these 

 s.' 1 ^. M. 



" I have added, the circulatory system, and particularly the 

 extremities of the vessels on the surface of the body. Like 

 the stomach, the action of the heart and arteries, and organs 

 of respiration, depends upon the energy of the brain, but then: 

 actions are performed with no very great variety ; they have a 

 certain latitude, but it does not extend wide ; and while the ac- 

 tion of the brain very often shows its effects in the organs of 

 voluntary motion, and in the alimentary canal, we know that 

 the circulating system is much less regularly affected ; only one 

 set of causes, viz. those which produce fever, are remarkable 

 for doing so. The organs of voluntary motion may be affected 

 with vigour or debility, with alacrity or sluggishness, while the 

 circulating system is not subject to the same states. If it is 

 subject to them, however, we find them especially in the ex- 

 tremities of the vessels, every where, but more especially in 

 those on the surface of the body. This has not been noticed 

 formerly. I consider the arteries as elastic chords constantly 

 in a state of tension between their two extremities ; it is obvious 

 that this tension is given by the fluids impelled by the concur- 

 rent action of the heart and larger vessels ; this action is pretty 

 steady, but not without some variety, at one time stronger and 

 at another weaker ; and the effects of the weaker action will be 

 perceived more or less according to the distance of the arteries 

 from the heart ; it will be most considerable, therefore, in their 

 extremities. There is a very frequent variety in the state of 

 the extreme arteries, they are frequently now fuller, now emp- 

 tier, sometimes greatly stretched out, at other times contracted 

 in then- length and diameter. Now the varied action of the 

 heart, for some particular reasons, varies the state of these arte- 

 ries more considerably on the surface of the body than in other 

 parts. Every increased action of the heart and arteries operates 

 more there, and produces sweat ten times for once that it in- 

 creases any internal secretion. The extreme arteries of the 

 surface are also connected with an organ of sense, and affected 



