328 



PHYSIOLOGY. 



centers, which in the emergency have risen in their functional abili- 

 ties. These minor vasoconstrictor centers exist in the spinal cord. 

 They may be excited in a reflex manner by means of strychnine (Ott 

 and Klapp). 



Upon destruction of the cord there follows a second fall 01 pres- 

 sure, with dilatation of the arterioles. 



Even when the spinal cord is destroyed in great part the tonus 

 of the vasomotor system is gradually regained so that heat and cold 

 applied to the skin have the same reaction take place as in normal 

 conditions of the vasomotor system. It is probably the circular muscles 

 of the blood-vessels which exert this tonic influence after destruction 

 of the spinal cord. 



Fig. 108. Carotid Pressure in Curarized Dog after Section of Medulla 

 A, and after Destruction of the Cord B. (GLEY.) 



Normal pressure was 120 millimeters. After section of the medulla it fell 

 to 66 (A). After destruction of spinal cord it fell to 40 millimeters (B). 



The vasoconstrictor center is in a state of permanent excitation, 

 which produces vascular tonus ; this is not , the case with the vaso- 

 dilator center. 



In a totally relaxed vascular system there is no possible circula- 

 tion the blood stands still. During extreme dilatation the heart re- 

 ceives but little blood, so that but very little is driven out of it during 

 systole. Hence the tonus of the blood-vessels is a necessary condition 

 for the circulation. 



The tonus of the veins is dependent upon the central nervous 

 system, and is quite as important as is that of the arteries. 



The vascular tonus is continually a seat of slight fluctuations, of 

 which the most important when depicted graphically constitute the 

 curves of Traube. The curves are the products of oscillations of the 

 vascular tonus. The oscillations are caused by variations in the 

 automatic excitation of the vasoconstrictor centers. (See "Traube- 

 Hering Curves.") 



