162 CIRCULATORY SYSTEM. 



with more places of contact with their parietes, put it more 

 under nervous influence than it is elsewhere. 



These vessels are not equally abundant in all the textures of 

 the body. Their quantity may be ascertained by the redness 

 which a part acquires by inflammation, as well as by fine injec- 

 tions: the latter proof is preferable, as, in the former, it is diffi- 

 cult to distinguish them from the extravasations which also 

 occur at the same time. Tke celebrated injections of Ruysch, 

 from their unusual minuteness, induced him to think that every 

 solid portion of the body was vascular, yet he admitted that 

 some portions were more vascular than others, thereby con- 

 ceding to his antagonists, that some points at least were not 

 formed by blood vessels. In the microscopical examinations 

 on living animals, for example, the frog, it is seen that in their 

 feet the smallest capillaries are separated by distinct intervals, 

 while in the mucous membrane of the lungs the finest needle 

 cannot have its point inserted without opening several of them.* 

 The younger an animal is, the more vascular are its parts: but, 

 on the contrary, as it advances in age, the proportion of parts 

 not susceptible of injection increases, while the capillaries di- 

 minish in number. In cold-blooded animals, it is very evident 

 that some of these capillaries, or arterio-venous communications, 

 are large enough to admit a file of several red globules abreast, 

 while others allow a single file only. As a general rule, their 

 diameter may be stated at from one to five globules of red 

 blood. t 



The nutrition of the body depends upon an alternation of 

 exhalation and of absorption; but it is still undetermined, whe- 

 ther there be any vessels whatever whose especial office is that 

 of exhalation, and which produce the several secretions and 

 exhalations. If there be such, they are generally designated 

 by the term exhalents, and their diameters are too small to 

 transmit the red globules >f blood ; their function is, consequent- 

 ly, to give passage to the serous particles only. This subject 

 has been much agitated Ly anatomists, and marshals the best 

 authorities on both sides. Among the distinguished advocates 

 in the affirmative, are Bocrhaave, Haller, and Bichat;. and op- 



* Boclard, Anat. Gen. t Beclurd, loc. eiL 



