318 DETAILED SYNTHESIS. 



893. Remark. IT is EVIDENT, FROM Fie. 204, that there is consider- 

 able space between the capillaries, filled with the substructures of the part ; 

 and the substances having made their exit from the capillaries, must 

 travel on to their destined place of use. So also must those that have ful- 

 filled their purpose travel back a greater or less distance to the free 

 moving current in the capillaries. 



894. Inf. Thus there is A CIRCULATION OUTSIDE THE CIRCULATION 

 IN THE BLOOD-VESSELS, a great commotion of substances moving to and 

 fro, the whole living body being a mass of activity ; and rubbing the body 

 and muscular contraction promotes this essential activity, this internal 

 circulation, in a still more important degree than they do the general or 

 external circulation in the Blood-vessels. 



895. Remark. IT is NOT SETTLED whether the capillaries are mere- 

 ly passive tubes or also adapted to secrete substances. The appearance 

 of the wall of the capillary is like that of secretory tissue ; and while it 

 acts, to a certain degree, passively, hi allowing certain substances to pass 

 through it, without change, it appears even then to have a selecting in- 

 fluence, or else it is exerted through the wall by the -structures outside. 

 In other cases, as hi the ganglia, there does not seem to be any reason 

 to believe the capillaries do not secrete the granular substance which ia 

 found around them, and is doubtless the material from which the cells 

 are formed. 



Veins. 



896. VEINS ARE continuations of the capillaries, the 

 wall of the latter being the basement membrane of the 

 former. It is lined with squamous (scale-like) or pave- 

 ment-cells, and is surrounded with sinewy fibres, partly 

 white or inelastic and partly yellow or elastic. By these 

 means the vessel is strengthened and thickened. In the 

 larger branches a few unstriated muscular fibrillaB are 

 found. 



897. The LARGER VEINS of the extremities, the 

 walls of the Trunk and Head, and the superficial veins 

 of the neck, ARE FURNISHED WITH VALVES. 



898. Ittus. The positions of some of these can be seen by rub- 

 bing the finger down upon the skin over the veins in the back of the 

 hand, since the blood is pressed against the valves, closing them, and dis- 



898. What is ? 894. Is there ? 895. What? Do capillaries secrete in gan- 

 glia ? 896. What ? 897. What ? 898. How positions of vein-valves seen ? 



