192 STRUCTURE OF CAPILLARIES. [BOOK i. 



through the cement lines, and especially at the point where the 

 cement lines of three or more cells meet together and where the 

 cement substance exists in larger amount than elsewhere. 



The size of the capillaries is variable. In some regions of the 

 body, for instance in the lungs, the capillaries are on the whole 

 wider than in other regions, for instance, the skin; and all the 

 minute vessels joining arteries to veins and possessing the struc- 

 tural features just described, that is, being true capillaries, will not 

 always have the same size even in the same region of the body ; the 

 artery may give rise to large capillaries which branch into small 

 capillaries, and these again may join into large capillaries before 

 uniting to form veins. Thus one capillary may be so narrow that a 

 single (mammalian) red corpuscle passes through it with difficulty, 

 whereas another capillary may be wide enough to afford room for 

 two or three such corpuscles to travel abreast. Besides this, the 

 same capillary, may, in the living body, vary in width from time 

 to time. At one moment, as when the entrance on the arterial 

 side is blocked, or when blood for some reason or another ceases to 

 flow into it, the capillary may be empty and collapsed, its walls in 

 contact, and its lumen abolished or nearly so ; and, in tissues taken 

 from the dead body and prepared for microscopical examination, 

 the capillaries are generally thus empty of blood and collapsed, so 

 that they can be seen with difficulty, appearing as they then do as 

 almost mere lines with swellings at intervals corresponding to the 

 nuclei of the constituent cells. At another time, as when blood 

 is flowing into it at high pressure, the capillary may be widely 

 distended. In the variations in calibre, the walls of the capillary 

 play a passive part ; the material of the epithelioid plates is 

 extensible and the pressure of the blood within the capillary 

 distends the walls, and the material being also elastic, the walls 

 shrink and collapse when the pressure is removed, being assisted 

 in this by the pressure of the lymph in the spaces outside the 

 capillary. But besides this, in a young animal, at all events, the 

 capillary wall is to a certain extent contractile; the epithelioid 

 cells, which then appear to contain a large amount of undifferen- 

 tiated protoplasm, seem able, under the influence of stimuli, to 

 change their form, passing from a longer and narrower shape to a 

 shorter and broader one, and thus influencing the calibre of the 

 tube of which they form the walls. How far such an active 

 change of form takes place in the capillaries of the adult body has 

 not yet been definitely determined. 



The structure of the capillary then seems adapted to two ends. 

 In the first place, its walls being permeable are adapted for 

 carrying out that important interchange between the blood and 

 tissue, which, as we have more than once said, takes place almost 

 exclusively in the capillary regions. In the second place, the 

 extensibility and elasticity of its walls permit it to adapt its calibre 

 to the amount and force with which the blood is flowing into it. 



