156 ANIMAL PHYSIOLOGY. 



of vessels. The "arrangement by which the blood-vessels 

 are enabled to furnish material for the nourishment of tex- 

 ture, and formation of secretions, while they also absorb 

 matters presented to them in both stomach and intestine, 

 is a very beautiful one. It is plain that blood, to which 

 variable amounts of chance ingredients had been added, 

 would be unfit for purposes of nutrition ; but the difficulty 

 is got over in this way, that the arterioles break up into 

 capillaries in the deep part of the mucous membrane, which 

 first supply the glandular structure, and then open into 

 venous radicles which take origin close to the surface; so 

 that the blood, in its purity, nourishes the glands, and does 

 not take up foreign matters until about to enter the veins. 



118. The passage of liquids through membranes is regulated 

 by physical laws of diffusion, which are closely connected 

 with capillary attraction. Just as gases diffuse according 

 to definite laws, so also do liquids. Their diffusion through 

 membranes or porous septa is called osmosis; or, inasmuch as 

 there are two currents in opposite directions wherever a 

 membrane separates two different fluids, the words endos- 

 mosis and exosmosis may be used to indicate the inward and 

 outward flow. If a piece of moist bladder be stretched 

 across a tube, and any saline solution introduced into the 

 vessel thus made, and the end of the tube be then dipped in 

 water, it will be found that in a short time a portion of the 

 solution has passed through into the water, while a larger 

 amount of water has passed into the tube, and raised the 

 height of the liquid within it. The same experiment may 

 be made with solutions of different sorts on the two sides of 

 the membrane. But the important points to note are, that 

 different solutions pass through in definite proportion to the 

 amount of any particular substance passing in the opposite 

 direction, and that while some substances diffuse. with facility, 

 others do so with difficulty. The substances which diffuse 

 easily are called crystalloids, while those which diffuse with 

 difficulty are called colloids (Graham). Thus albumen in its 

 ordinary condition is a colloid, but when converted into pep- 

 tone it becomes crystalloid. It is in consequence of endos- 

 mosis that, when water is added to blood, the red corpuscles 

 become swollen and spherical. The substance in which the 



