220 A TEXTBOOK OF PHYSIOLOGY 



By the application of a piece of ice to the tongue of a frog, vaso- 

 dilatation can be converted into constriction. The arteries become 

 narrow, the tongue pale. The eye has difficulty in finding any except 

 the larger vessels. Few capillaries appear to contain blood, and 

 where a considerable quantity of blood is still present, as in the arteries 

 and veins, the flow is tardy, and even in the arteries the individual 

 corpuscles can now generally be recognized. 



In a warm-blooded animal, the results of exposure to an irritant 

 are much more rapidly established. After exposure of a rabbit's 

 ear to water at 55 C., the blood is altogether unable to penetrate 

 the arteries. A change has taken place in the relations between the 

 blood and the vessel wall as regards friction and adhesiveness, and 

 thus complete stasis of the circulation arises. If the change be less 

 intense, the porosity of the vessel is affected, and a quantitative and 

 qualitative change in the transudation from the capillaries ensues. 

 The rabbit's ear may be entirely separated from the body, with excep- 

 tion of the central artery and vein. After section of all the vaso- 

 motor nerves by this means, vascular dilatation is greatly increased 

 by rubbing the ear, and all the phenomena of inflammation occur 

 after the application of an irritant. We have here to deal, not with a 

 nervous mechanism, but with a change of the vessel wall. The circu- 

 lation through the capillaries is possible only so long as the vessel wall 

 is in the normal physico-chemical condition which characterizes the 

 living state. 



The corpuscles continue to move through the capillaries for some 

 seconds, or even minutes in a few of the capillaries, after the bulbus 

 arteriosus has been ligated ; they run faster on pressing or moving the 

 leg. Observations of this kind show how immeasurably slight a 

 difference of pressure is required to produce a flow in the capillaries. On 

 clenching the fist the capillaries of the hand blanch. By the ceaseless 

 muscular movements and changes of posture of the living mobile animal 

 the capillary pressure is kept in the skin approximately the same as 

 the atmosphere, for whenever the blood is thus pressed out of them 

 into the veins the pressure does not become positive in the capillaries 

 till they fill again. 



In the intestinal wall the blood is similarly expressed by the 

 muscular contractions of the gut. In encapsulated organs, such as the 

 glands, on the other hand, the capillary pressure may rise with the 

 secretory pressure up towards the arterial pressure. This is the case 

 in the salivary gland when the secretion is made to take place 

 against pressure. Secreting cells are confined by limiting membranes, 

 membranae proprise, tough and homogeneous, but of great tenuity. 

 These membranes, while allowing the protoplasm of gland cells 

 or muscle plasma to imbibe fluid from the capillaries, limit the 

 expansion produced by intracellular forces. Thus the salivary 

 glands may secrete saliva at a pressure greater than arterial 

 pressure, and the blood continue to flow through the gland. 

 The expansion of the alveoli is limited, so that it narrows and 

 does not shut up the veins (see Fig. 119c). The result is 



