220 BIOLOGY. [Book ir. 



strong moral impressions, that emotions produce the same 

 effects ; the change of the coloration of the cheeks is a familiar 

 sign of strong emotions, especially in the young and in women ; 

 that is to say, when the impressionability is the more intense and 

 the reflex actions the more easy. The changes of coloration are 

 far from coinciding always with the abnormal cardiacal pulsa- 

 tions ; their physiological reason is a reflex vaso-motory action. 



In physiology and in pathology the ofiice of the vaso-motory 

 nerves and of the capillaries is .enormous. The variations in the 

 calibre of the small vessels regulate the distribution of the blood 

 even in the innermost part of the tissues, and thus act direct on 

 nutrition, and consequently on the operation of the organs. As 

 a general rule any moderate dilatation of the capillaries of 

 an organ has for efiect the superactivity of that organ, whatever 

 it may be, gland, muscle, brain, and so on. On the contrary, 

 every contraction too great brings with it a, nutritive retardment 

 and a functional diminution, forasmuch as it lessens the quantity 

 of arterial blood passing in a given time within range of the 

 anatomical elements, and consequently the proportion of liquid 

 and gaseous exchanges of which the capillaries are the principal 

 seat. 



After having, by circulating in the capillaries, supplied the 

 anatomical elements with assimilable materials, and retaken the 

 waste substances ; after having yiielded at the same time the 

 oxygen needful to the nutritive chemical transformations, and 

 received in return the carbonic acid, product of the vital com- 

 bustions, the arterial blood, formerly vermilion, becomes venous 

 blood; black blood, and passes from the capillaries into vessels 

 more and more voluminous. These vessels are at first similar 

 to the ultimate arterioles which pass into the capillaries ; then 

 little by little their muscular and elastic tunics grow thinner ; 

 some longitudinal contractile fibres cross the circular fibres ; the 

 vascular walls grow less rigid ; the vessels have become venous. 

 They go on thus anastomosing each other, augmenting always 

 in calibre, to reduce themselves in the mammifers to two great 



