^^ ON T»E FUNC'WONS OF THE HEART A>N» ARTERIES. 



!h« inference i-ous system agree uniformly, in ail its parts, with the mea- 



uotaff^ect d by s,^j.,.,g vv{iioh I have laid down ; but the trut'i ol" the inference 



wv vamtK<ns 



that may be is not affected by these variations. For example, there may 



supposed m perhaps be some arteries communicatiny- with veins, of 

 the (Jimt-flaions. , . , , , , , , . i i i ^ 



which the di'ameter exceeds the eleven nurKlredths of an 



inch; but there are certainly many others, whi(.h are much 

 more minute; and the blood, or its more !i(}nid parts, pass- 

 ing through these more slowly, it must move more ra^jidly 

 iti the former, so that the resistance u^ay in all be equal to 

 the pressure, and the mean velocity mav stiil remain such 

 as is determined by the quantity of blood passing throiigh 

 Siie of the the aorta. There is indeed some uncertainty in the mea- 

 globulesofthe gQ^e of the globules of the blood, which 1 have made the 



blued, ^ • n 1 ^• ■ L' \ • • 1 T 1 



basis or the dimensions ot the minute arteries: and 1 have 

 reason to thi'nk, that instead of ^-^W of an inch, their great- 

 est diameter does not exceed -j-^st^, or even -s-o-Vtr' the ge- 

 neral results of the investigation are not however affected 

 by this difference : it will only require us to suppose the 

 bubdivisions somewhat more numerous, and the branches 

 shorter. 



These are the principal circumstances, which require to 

 be considered, with respect to the simple transmission of the 

 blood through the arteries into the veins, without regard to 

 the alternate motions of the heart, and to the elastic and 

 Nature and ve- ™*^''*^"^^'^' P''^'*^'** ^^ the vessels. I shall next examine the 

 lociiy of the nature and velocity of the propagation of the pulse. The 

 thepulse. successive transmission of the pulsations of the heart, 

 through the length of the arteries, is so analogous to the 

 motion of the waves on the surface of water, or to that of a 

 sound transmitted through the air, that the same calcula- 

 tions will serve for determining the principal affections of all 

 these kindt of motion ; and if the water, which is agitated 

 by waves, is supposed to flow at the same time in a conti- 

 nued stream, and the air which conveys a sound to be car- 

 ried forwards also in the form of a wind, the similitude will 

 Elasticity of be still stronger. The coats of the arteries may perhaps be 

 thecoatsofthe ., , ^.^, ^ , . ,. , , • 



arteries. consitiered, without much inaccuracy, as perfectly elastic; 



that is, as producing a force proportional to the degree in 

 which they are extended beyond their natural dimensions; 

 but it is not iiii possible, that there may be some bodies in 



nature^ 



