332 INSTANCES OF VASO-MOTOR ACTION. [BOOK i. 



general, but we may here quote one or two characteristic ones. 

 "Blushing" is one. Nervous impulses started in some parts of the 

 brain by an emotion produce a powerful inhibition of that part of 

 the medullary vaso-motor centre which governs the vascular areas 

 of the head supplied by the cervical sympathetic, and hence has an 

 effect on the vaso-motor fibres of the cervical sympathetic almost 

 exactly the same as that produced by section of the nerve. In 

 consequence the muscular walls of the arteries of the head and 

 face relax, the arteries dilate and the whole region becomes 

 suffused. Sometimes an emotion gives rise not to blushing, but to 

 the opposite effect, viz. to pallor. 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 con- 

 striction, like the dilation 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 constricted, 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 augmen- 

 tation of constriction in the one case and inhibition in the other ; 

 though possibly some slight effect is produced by the direct action 

 of the cold or heat on the vessels of the skin simply. Moreover 

 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 speak, 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 blood vessels 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 dilation 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 im- 

 pulses and sometimes 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 meta- 

 bolism of tissue which is the basis of that activity is assisted by 

 a more generous flow of blood through the tissue. 



