86 THE CAPILLARY TISSUE. 



vessels. The skin of man also shows that there is no point 

 on its surface that can be pricked without drawing blood. 

 The mucous membranes are as abundantly, if not more so, 

 supplied with capillary vessels than the skin. The cartil- 

 ages, epidermis and hair are not penetrated by injection, 

 and show no blood-vessels whatever. The ligamentous 

 structure, the dura-mater, periosteum, &c., have few blood- 

 vessels. 



The question here arises, if these parts have few or no 

 blood-vessels, how do they grow and how are they nour- 

 ished? The answer to this question led Bichat to believe 

 and assert that there was another kind of capillaries be- 

 sides the sanguineous, which carried the serous or colorless 

 portions of the blood to the white structures, and which 

 he called exhalants. The existence of exhalant vessels has 

 never been satisfactorily demonstrated, though admitted 

 by many to exist. The lymphatic capillaries come under 

 the same head, both of them being considered equally ne- 

 cessary for development and support to the white organs, 

 as the sanguineous capillaries are to all other parts. 



Various agents are capable of acting on the capillaries, 

 producing contraction and dilatation, and these may be 

 mechanical, chemical or mental. 



The principal functions of the capillaries are those of 

 nutrition and secretion, and their independent action, ac- 

 cording to Bichat, that of carrying on their own circula- 

 tion without the heart, and also of giving the impulsive 

 power to the veins. 



This independent action in the capillary system, if one 

 part of the body can in any sense be said to be indepen- 

 dent of every other part, seems most likely true, as there 

 are some animals low in the scale of being, which are 

 entirely without heart and blood vessels, and possess no 

 other kind of circulation but the capillary, which is re- 

 garded as the primitive or fundamental circulation. 



