164 CIRCULATORY SYSTEM. 



different secretions. The question oL the exhalents being a 

 distinct set of vessels, does not, however, appear to be one of 

 much consequence ; because, if they do exist, they must be very 

 short and very small; and the assumption of their existence does 

 not throw any light upon the function of secretion. For the 

 latter is still an incomprehensible vital process, and as far as 

 we have any idea about it, it is quite as easy to conceive of its 

 being performed in the pa*ietes of the capillaries, as in the 

 mouths of a distinct set of vessels, whose length is too short to 

 admit of an estimate. 



Besides the supposed existence of a general system of exha- 

 lent vessels, some anatomists have thought that there was a spe- 

 cies of them acting particularly as nutritive vessels. According 

 to Boerhaave,everypart must, therefore, be vascular. Mascag- 

 ni thought that the extreme arterial ramifications are not only 

 furnished with exhaling, but also with nutritive porosities; and 

 that there are every where orifices of absorbing vessels, to con- 

 tain the nutritive molecules. The theories of Bichat and of 

 Prochaska, do not differ materially from the latter. Whatever 

 may be the mode of existence, and the route of nutriment to the 

 several parts of the body, the operations involved are entirely 

 too subtle even for microscopic observation. We, therefore, 

 can only understand, in a general way, that the blood vessels 

 deposite, and the lymphatics absorb, by invisible avenues in the 

 cellular substance, the molecules of composition and of decom- 

 position in our organs.* It is to this power that the name of 

 vital force has been given, and especially that of the force of 

 formation, (nisus formativus.) 



The arteries, though commonly said to be cylindrical canals, 

 are not exactly so, but, as they recede from the heart, increase 

 somewhat in diameter, even where they do not send off any 

 branches. In this way the arteries of the umbilical chord are 

 evidently larger as they get nearer the placenta; and the sper- 

 matic arteries of a bull as they get nearer to the testicle. Ob- 

 servations made on the carotid arteries of the camel, and of the 

 swan, by Mr. Hunter,f tend to prove the same disposition in 

 them. It is probable that the rule extends to all arteries through- 

 out the system, but it cannot be ascertained with so much cer- 



* Beclurd, loc. cit f On the Blood and Inflammation. 



