THE BLOOD-VESSELS. 



145 



body ' capillary plexuses are interposed between arteries and 

 veins, which constitute a series of conveying and returning 

 tubes. Thereby the direct continuity of these blood-channels 

 is established. 



It is in these intermediate territories, and in them only, that 

 the blood serves its true function of giving and taking. True 

 markets of exchange, then, these capillary districts, where the 

 system is supplied with new material, and in 

 return gets rid of useless or even deleterious 

 by-products of tissue-life. Hence, the para- 

 mount importance of these vessels in the 

 maintenance of life and health. Hence, also, 

 the direct practical utility of knowing their 

 minute anatoni}^ and physiological dignity. 

 Every practitioner of medicine will see the 

 important relation this branch of histology 

 holds to pathology, and therefore to thera- 

 peutics. At the same time we should not 

 forget that the role played b} r the capillaries 

 in the system is normally due to the inherent 

 mechanical and physical properties of a fine- 

 ly elastic animal membrane, rather than to 

 any specific action of their cellular constitu- 

 ents. 



Robin, following Henle's example, dis- 

 tinguishes several varieties of these vessels. It 

 seems to me proper to limit the term capil- 

 laries to those minute tubules which are 

 entirely devoid of muscular elements. This 

 corresponds to the classification adopted by 

 Virchow, Kolliker, Eberth, Ranvier, Frey, 

 and others. It is the one therefore that has 

 generally been accepted, and is both simple 

 and logical. 



The diameter of these tubules varies from 

 0.0045 to 0.0115 mm. Their structure is readily understood. 

 Examined in the living animal with a high power, we see mere- 

 ly a delicate, hyaline, double-contoured membrane, having an 



1 Hoyer has shown that a direct communication of arterioles with venules occurs 

 normally in the tips of the fingers, the matrix of the nails, the tip of the nose, and 

 various other parts. 

 10 



Pio. 59. A rather Inrce 

 capillary from the hyaloid 

 of the frog, presenting a 

 membranous and nucleated 

 tunica adventitia. Eberth. 



