CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 533 



diaphragm ; the increased capillary pressure thus caused, especially 

 perhaps that of the abdominal viscera, gives rise to an increased 

 transudation and so to an increased flow. We need, however, not 

 dwell further on this. There can be no doubt that increase of 

 capillary pressure due to increased venous pressure is especially 

 potent in causing increased transudation. 



So far the facts are in favour of the process of transudation 

 being merely an ordinary nitration. But we have now to ask 

 the question, May the capillary pressure be increased without any 

 increase of transudation resulting ? 



An affirmative answer to this question seems to be afforded by 

 such instances as the behaviour of the submaxillary gland, when 

 the chorda tympani is stimulated while the gland is under the 

 influence of atropin. The vessels are as we have seen ( 227) 

 dilated, the capillary pressure is increased, and yet there is no 

 secretion. At the same time there is no accumulation of lymph 

 in the connective-tissue spaces of the gland, and no marked in- 

 crease in the lymph flowing away along the lymphatics leaving 

 the gland. We infer that, under the atropin, although the 

 capillary pressure is increased, no increase of transudation takes 

 place. Owing to the complexity of the act of secretion we may 

 perhaps hesitate to lay too much stress on this instance; but it 

 and others like it seem to indicate that under certain conditions 

 increase of pressure does not entail increase of transudation. 



We may now ask a further question, May the transudation be 

 increased without any corresponding increase of pressure in the 

 capillaries ? Certain substances such as peptone (albumose), extract 

 of leeches' heads, extract of crabs' muscles, and others, when 

 injected into the blood of an animal (dog) give rise to such an increase 

 in the flow of lymph along the thoracic duct that they have been 

 spoken of as " lymphagogues." Now though these substances do 

 tend to dilate the small arteries and thus to increase the capillary 

 pressure (this not being compensated by the lowering of general 

 blood-pressure resulting from the dilation of the arteries), they 

 may be so injected as not at all to produce this effect, and yet 

 their influence in increasing the flow of lymph, that is in increasing 

 transudation, continues. These substances have a marked effect 

 on the clotting of blood ( 22), and that effect appears to be 

 produced, in part at least, by the substances acting in some way on 

 the vascular walls. We seem to have a right to infer that they 

 promote transudation, independent of changes of pressure, by 

 some action on the vascular walls. The following is an instance of 

 a similar action of another substance. Though a ligature of a vein 

 generally leads to increased transudation, and so to oedema, it does 

 not under all circumstances do so. When the femoral vein of a 

 dog for instance is ligatured, it may happen that no oedema results. 

 But if under these circumstances a proteid substance of the nature 

 of nucleo-albumin, obtained from cellular structures, be injected 



