290 THE TISSUES AND MECHANISMS OF DIGESTION. 



vessel becomes much increased, though, of course, remaining lower than 

 the general arterial pressure. Moreover, during digestion, peristaltic move- 

 ments of the muscular coats of the alimentary canal are, as we have seen, 

 active; and these movements, serving as aids to the circulation (see 110), 

 help to increase the portal flow. Further, the spleen, as we shall see in 

 speaking of that organ, is in many animals richly provided with plain mus- 

 cular fibres, and in such cases seems, especially during digestion, to act as a 

 muscular pump driving the blood onward, with increased vigor, along the 

 splenic veins to the liver. So that even were the liver not connected with 

 the central nervous system by a single nervous tie, the tide of blood through 

 the liver would ebb and flow according to the abse-nce or presence of food 

 in the alimentary canal. 



An increase of blood-supply does not, of course, necessarily mean an 

 increase of secretory activity. As we have seen, 197, in the presence of 

 atropine the secretion of saliva may stand still in spite of dilated blood- 

 vessels and the consequent rush of blood ; but we may safely assert that, 

 other things being equal, a fuller blood-supply is favorable to activity. 

 Apparently a mere change in the quantity of blood bathing an alveolus 

 will not start in the cells the changes which constitute the act of secretion, 

 any more than an increase in the blood bathing a muscular fibre will neces- 

 sarily set going a contraction ; but unless there be some counteracting 

 influence at work, a fuller and richer lymph around a cell will naturally 

 lead to the cell taking up more material from the lymph, and so will increase 

 the cell's store of energy. Hence, especially in the hepatic cell, which 

 appears to be always at work, always undergoing metabolism of such a 

 kind as to give rise to bile, we might fairly expect the greater flow through 

 the portal vein to quicken the flow through the bile-duct. 



And, as a matter of fact, we do find vaso-constrictor action dominant 

 over the secretion. In the various experiments which have been made to 

 ascertain the action of the nervous system on the secretion of bile, it has 

 always been found that stimulation of the medulla oblongata, or of the 

 spinal cord, or of the abdominal splanchnic nerves, stops, or at least 

 checks the flow of bile. Now the effect of these stimulations is, as we 

 have already seen more than once, a powerful constricting action on the 

 abdominal bloodvessels ; by such stimulation the blood-supply of the liver 

 is materially diminished, and in consequence the secretory activity is slack- 

 ened or arrested. 



But there is something besides the mere quantity of blood to be consid- 

 ered in this relation. The blood which passes from the alimentary canal at 

 rest is ordinary venous blood, laden simply with carbonic acid and the ordi- 

 nary products of the metabolism of the muscular and mucous coats of the 

 canal. When digestion is going on the portal blood is laden, as we shall 

 see, with some, at all events, of the products of digestion, with sugar prob- 

 ably and with various proteid bodies. And it is quite possible or even prob- 

 able that some of these bodies in the portal blood reaching the hepatic cells, 

 stir them up to secretory activity ; indeed, this view may be regarded as sup- 

 ported by the fact that 'proteid food increases the quantity of bile secreted, 

 whereas fatty food, which as we shall see passes chiefly, if not wholly, not 

 by the portal vein but by the lymphatics, and which is probably largely 

 disposed of in some way or other before it can reach the liver, has no such 

 effect. 



Hence we may infer that at all events the second increase of the flow 

 of bile which occurs during the later stages of digestion, may be to a 

 large extent the direct effect of blood, laden with digestive products, 

 passing from the stomach and intestines, especially the latter, to the liver 



