292 DISEASES Sect. XXXII. e. 2. 



and thefe four mechanic properties are as different from each 

 other as the various chemical ones, which are adapted to the 

 numerous glands, and to the other organs of fenfe. 



2. The momentum of the progreflive particles of blood is 

 compounded of their velocity and their quantity of matter: hence 

 whntever circumflances diminifh either of thefe without propor- 

 tionally increafing the other, and without fuperadding either of 

 the general ftimuli of heat or diftention, will tend to produce a 

 quiefccnce of the arterial fyftem, and from thence of all the 

 other irritative motions, which are connected with it. 



Hence in all thofe conflitutions or difeafes where the blood 

 contains a greater proportion of ferum, which is the lighted part 

 of its compofition, the pulfations of the arteries are weaker, as in 

 nervous fevers, chlorofis, and hyfleric complaints ; for in thefe 

 cafes the momentum of the progreflive particles of blood is lefs ; 

 and hence, where the denfer parts of its compofition abound, as 

 the red part of it, or the coagulable lymph, the arterial pulfations 

 are ftronger •, as in thofe of robuft health, and in inflammatory 

 difeafes. 



That this flimulus of the momentum of the particles of the 

 circulating fluid is of the greateft confequence to the arterial 

 action, appears from the experiment of injecting air into the 

 blood veflels, which feems to deflroy animal life from the 

 want of this flimulus of momentum ; for the diftention of the 

 arteries is not diminifhed by it, it poflefles no corrofive acrimony, 

 and is lefs liable to repafs the valves than the blood itfelf ; fince 

 air-valves in all machinery require much lefs accuracy of con- 

 ftruclion than thofe which are oppofed to water. 



3. One method of increafing the velocity of the blood, and in 

 confequence the momentum of its particles, is by the exercife 

 of the body, or by the friction of its furface •, fo, on the contra- 

 ry, too great indolence contributes to decreafe this flimulus of 

 the momentum of the particles of the circulating blood, and thus 

 tends to induce quiefcence ; as is feen in hyiteric cafes, and 

 chlorofis, and the other difeafes of fedentary people. 



4. The velocity of the particles of the blood in certain cir- 

 cumftances is increafed by venefeclion, which, by removing a 

 part of it, diminifhes the refiflance to the motion of the other 

 part, and hence the momentum of the particles of it is increaf- 

 ed. This may be eafily underflood by confidering it in the ex- 

 treme, fince, if the refiflance was greatly increafed, fo as to over- 

 come the propelling power, there could be no velocity, and in 

 confequence no momentum at all. From this circumftance 

 arifes that curious phenomenon, the truth of which I have been 



>re than once witnefs to, that venefedion will often inflanta- 



neoufly 



