THE CAPILLARY CIRCULATION. 231 



In a great number of cases this has quite a different cause, being due to a 

 sudden diminution or even temporary arrest of the heart's beats ; but in 

 some cases it may occur without any change in the beat of the heart, and is 

 then due to a condition the very converse of that of blushing, that is, to an 

 increased arterial constriction ; and this increased constriction, like the dila- 

 tation of blushing, is effected through the agency of the central nervous 

 system and the cervical sympathetic. 



The vascular condition of the skin at large affords another instance. 

 When the temperature of the air is low the vessels of the skin are con- 

 stricted and the skin is pale ; when the temperature of the air is high the 

 vessels of the skin are dilated and the skin is red and flushed. In both 

 these cases the effect is mainly a reflex one, it being the central nervous 

 system which brings about augmentation of constriction in the one case and 

 inhibition in the other, though possibly some slight effect is reproduced by 

 the direct action of the cold or heat on the vessels of the skin simply. More- 

 over, the vascular changes in the skin are accompanied by corresponding 

 vascular changes in the viscera (chiefly abdominal) of a reverse kind. When 

 the vessels of the skin are dilated those of the viscera are constricted, and 

 vice versa, so that a considerable portion of the whole blood ebbs and flows, 

 so to spaak, according to circumstances from skin to viscera and from viscera 

 to skin. By these changes, as we shall see later on, the maintenance of the 

 normal temperature of the body is in large measure secured. 



When food is placed in the mouth the bloodvessels of the salivary glands, 

 as we have seen, are flushed with blood as an adjuvant to the secretion of 

 digestive fluid ; and as the food passes along the alimentary canal, each 

 section in turn, with the glandular appendages belonging to it, welcomes its 

 advent by flushing with blood, the dilatation being sometimes, as in the case 

 of the salivary gland, the result of the activity chiefly of vaso-dilator fibres, 

 but sometimes the result of the cessation of constrictor impulses and some- 

 times the result of the two combined. So also when the kidney secretes 

 urine, its vessels become dilated, and in general, wherever functional activity 

 comes into play, the metabolism of tissue which is the basis of that activity 

 is assisted by a more generous flow of blood through the tissue. 



166. Vasomotor nerves of the veins. Although the veins are provided 

 with muscular fibres and are distinctly contractile, and although rhythmic 

 variations of calibre due to contractions may be seen in the great veins 

 opening into the heart, in the veins of the bat's wing, and elsewhere, and 

 similar rhythmic variations, also possibly due to active rhythmic contrac- 

 tions, but possibly also of an entirely passive nature, have been observed in 

 the portal veins, very little is known of any nervous arrangements govern- 

 ing the veins. When in the frog the brain and spinal cord are destroyed, 

 very little blood comes back to the heart as compared with the normal 

 supply, and the heart in consequence appears almost bloodless and beats 

 feebly. This has been, by some, regarded as more than can be accounted for 

 by mere loss of arterial tone, and accordingly interpreted as indicating the 

 existence of a normal tone in the veins dependent on the central nervous 

 system. When the latter is destroyed, the veins become abnormally distended 

 and a large quantity of blood becomes lodged and hidden, as it were, in them. 



THE CAPILLARY CIRCULATION. 



167. We have already some time back ( 106) mentioned some of the 

 salient features of the circulation through the capillaries, viz., the difficult 

 passage of the corpuscles (generally in single file, though sometimes in the 

 larger channels two or more abreast) and plasma through the narrow chan- 



