PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES. 281 



every direction, distributing blood to the parts, as their phys- 

 iological necessities demand. This inosculation is peculiar 

 to these vessels, and the plexus is rich in the tissues, as a gen- 

 eral rule, in proportion to the activity of their nutrition. 

 Though their arrangement presents certain differences in 

 different organs, the capillary vessels have everywhere the 

 same general characteristics, the most prominent of which 

 are uniform diameter and absence of any positive direction. 



The network thus formed is very rich in the substance 

 of the glands, and in the organs of absorption ; but the ves- 

 sels are only distended with blood during the physiological 

 activity of these parts. In the lungs the meshes are partic- 

 ularly close. In other parts the vessels are not so abundant, 

 presenting great variations in different tissues. In the mus- 

 cles and nerves, in which nutrition is very active, the supply 

 is much more abundant than in other parts, like nbro-serous 

 membranes, tendons, etc., whose functions are rather passive. 1 

 In none of the tissues do we find capillaries penetrating the 

 anatomical elements, as the ultimate muscular or nervous 

 fibres. Some tissues receive no blood, at least they contain 

 no vessels which are capable of carrying red blood, and are 

 nourished by imbibition of the nutrient plasma of the circu- 

 lating fluid. Examples of these, which are called extra-vas- 

 cular, are cartilage, nails, hair, etc. 



The foregoing anatomical sketch gives an idea of how 

 near the blood is brought to the tissues in the capillary sys- 

 tem, and how, once conveyed there by the arteries, and the 

 supply regulated by the action of the muscular coat of the 

 smaller vessels, the blood is distributed for the purposes of 

 nutrition, secretion, absorption, exhalation, or whatever func- 



1 The arrangement of the capillaries in different tissues and organs has gen- 

 erally been ascertained by minute injections. In studying injected preparations, 

 however, it must be borne in mind that when injected, the elastic and yielding 

 vessels are distended to their extreme capacity, and the capillaries, therefore, 

 occupy a space much greater than is natural. In injections of the liver, for ex- 

 ample, the capillaries seem to constitute the bulk of the organ, and we are at a 

 loss to understand how the cells, ducts, etc., find place between their meshes. 



