320 THE TISSUES AND MECHANISMS OF DIGESTION. 



the results of one or other of these physical processes ? Diffusion by itself 

 will not account for the results ; for the proteids of the blood-plasma are 

 iudiffusible or very nearly so, and yet the lymph contains a considerable 

 quantity of these proteids. We have no satisfactory knowledge of the exact 

 composition of lymph as it exists in the lymph-spaces. In the lymph of the 

 larger lymph-trunks the diffusible saline substances are present in about the 

 same proportion, and the indiffusible proteids to about or less than half as 

 much as in blood- serum ; and we may perhaps assume that the lymph in the 

 lymph-spaces contains relatively less proteids, but has otherwise the same 

 composition as blood-plasma. Mere diffusion would not give rise to a fluid 

 of such a nature. Can we speak of transudation, then, as a filtration ? The 

 blood is undoubtedly flowing through the capillaries and other small vessels 

 under a certain pressure ; we have seen ( 105) that the pressure is roughly 

 speaking about 30 mm. Hg. ; and it would be possible to select such a filter 

 or porous partition as would at about this pressure permit the passage of a 

 certain quantity of the inorganic and crystalline constituents of blood-plasma 

 to pass through in company with a relatively smaller quantity of the proteids 

 and a large quantity of the water, the red and white corpuscles being ex- 

 cluded. Such a filtrate would be more or less of the nature of lymph ; and 

 so far we might be justified in speaking of the transudation of lymph as a 

 process of filtration. But the transit through the living wall of the blood- 

 vessel is affected by circumstances in a manner so different from the manner 

 in which the same circumstances affect a transit through an ordinary lifeless 

 filter, that we gain but little and may be led into error by speaking of the 

 process as a filtration. Substances in solution or otherwise, pass through a 

 filter when the pressure is sufficient to drive them through the passages 

 furnished by the interstices existing in the substance of the filter. In the 

 case of an ordinary filter the substance of the filter is within limits perma- 

 nent, and the passages correspondingly constant. The living wall of a 

 capillary, however, is not a constant unchanging thing. The epithelioid 

 plates and other elements which constitute it are alive, and being alive are 

 continually undergoing change and are especially subject to change ; more- 

 over, as we have seen ( 22, 23), the vascular walls appear to be continu- 

 ally acting upon and being acted upon by the blood. Hence a change in 

 the blood tends to cause changes in them ; and these changes may materially 

 affect in one direction or another their action as filters. In an ordinary filter 

 increase of pressure necessarily entails increase of filtration; in a living 

 filter it may or may not, and the same increase of pressure may according 

 to circumstances produce very different results as regards the transudation 

 of lymph. 



Thus it seems reasonable to suppose, as we have suggested ( 197), that, 

 other things being the same, an increase of blood-pressure should necessarily 

 increase the transudation of lymph. Hence when a small artery dilates, 

 since the pressure in the still smaller branches and capillaries of that artery 

 is, as we have more than once pointed out, increased, more lymph appears in 

 the lymph-spaces ; indeed it is one of the main purposes of the widening of 

 small arteries to supply the elements of the tissue with more lymph, that is, 

 with more food. But it does not therefore follow that under all circum- 

 stances widening of the artery should increase the passage of lymph ; some- 

 thing may occur to counteract the natural effect of the increased pressure in 

 the bloodvessels. An instance of this seems to be afforded by the case of 

 the submaxillary gland, when the chorda nerve is stimulated while the gland 

 is under the influence of atropine. As we have seen, though the arteries 

 dilate, no secretion takes place ; and we cannot explain the absence of a flow 

 into the alveoli by supposing that the extra amount of lymph which would 



