THE CIRCULATION. 329 



The vasoconstrictor center is excited during dyspnoea and as- 

 phyxia. This occurs on account of the accumulation of carbonic acid 

 in the blood. This action explains why the arteries in the cadaver 

 are free from blood. Strychnine, nicotine, and Calabar bean also 

 excite the vasoconstrictor center. 



Advantages of Vessel Innervation. By reason of vascular tonic- 

 ity the diameters of the vessels are a trifle too small to contain all 

 the blood ; so that the vascular walls are obliged to dilate. The result 

 is pressure and circulation of the blood. 



When various organs and parts of the body are in activity they 

 require an excess of blood. This surplus is furnished by a dilatation 

 of the capillaries of the part. Ludwig compared the vasomotor cen- 

 ters to turn-cocks in a great city. They turn off the water-supply 

 from one district and at the same time turn it on in another. 



As previously stated, the cutaneous circulation regulates the losses 

 of heat. 



When, from the influence of cold, the capillaries of the skin are 

 narrowed, the internal organs are congested. Under the action of 

 heat the skin is congested and the internal organs made anaamic. 

 This increase in the blood-supply in those parts where needed has 

 been ingeniously demonstrated by Mosso. He placed a man upon 

 a very large board which was most delicately balanced at its center. 

 By use of it he demonstrated that whenever the man began to think 

 the increased blood-supply in .his brain caused the head to go down 

 and the heels to rise up. 



In muscular work the vessels of the muscles and skin are dilated, 

 whilst the intestinal arterioles supplied by the splanchnics are con- 

 tracted ; hence the arterial tension is increased. 



About one-half the blood in the body is stored up in the thorax 

 and abdomen ; hence it can quickly supply a large amount of blood to 

 any organ in activity which needs it. The kidney when compared with 

 other organs receives a large quantity of blood for its important duties 

 of removing the waste matter from the body. 



Vasomotor Reflexes. The afferent nerves stand in relation to the 

 main vasomotor center in such a way that they can either excite or 

 inhibit its activity. A pressor nerve is one which when stimulated 

 reflexly excites the vasomotor center and produces a rise of blood- 

 pressure. A depressor nerve is an afferent nerve which when 

 (inlated inhibits the activity of the main vasomotor center. In the 

 rent nerves we have these two kinds of fibers pressor and depres- 

 The pressor fibers are especially found in the nerves of the skin. 



