33 2 TEXT-BOOK OF PHYSIOLOGY. 



terminating at a point four millimeters above the tip of the calamus 

 scriptorius. 



A transection of the medulla above the upper limit of this area is 

 without effect on the blood-pressure. A similar section below it, 

 however, is at once followed by vascular dilatation, a loss of vascular 

 tone, and a general fall of blood-pressure. Subsequent stimulation 

 of the peripheral end of the divided medulla, the animal being curar- 

 ized and artificial respiration maintained, will give rise to a marked 

 contraction of the blood-vessels and a rise of blood-pressure up to 

 and far beyond the normal value. 



If the experimental lesion is limited to the area mentioned in the 

 . foregoing paragraph, the vascular dilatation passes away after a 

 time, the blood-vessels regain their normal tone, and the pressure 

 again rises. These facts indicate that the area is to be regarded as 

 the general vaso-motor (constrictor] center which maintains the tonus 

 of the blood-vessels through its dominating influence over the vaso- 

 motor centers in the cord, the latter acting in a subsidiary manner to 

 the former. The nerve-fibers which transmit the regulative nerve 

 impulses from the general to the subsidiary centers are found in the 

 lateral columns of the cord^ There is no evidence for the existence 

 of a general vaso-dilatator center in the medulla^? Since the blood- 

 vessels maintain a more or less constant tone, it is assumed that the 

 vaso-motor centers are in a state of continuous activity. In how far, 

 however, this activity is the result of chemic changes between the 

 cells and the surrounding lymph and blood, or the result of con- 

 stantly arriving nerve impulses reflected from the periphery or from 

 higher regions of the nervous system, is not readily determinable. 

 Both factors are probably involved. 



Direct Stimulation of the Vaso-motor Centers. The general 

 vaso-motor (constrictor) center at least is markedly influenced by 

 the quantity and quality of blood and lymph circulating around and 

 through it. If the blood-supply to the medulla and associated 

 structures be diminished by compression of the carotid arteries, the 

 activity of the center is at once increased, as shown by increased vas- 

 cular contraction and a rise of pressure. Restoration of the blood- 

 supply is followed by a return of the center to its normal degree of 

 activity. Increased blood-supply, as in cerebral hyperemia, is at- 

 tended by a fall in blood-pressure indicating a decrease in the activity 

 of the center. A diminution in the percentage of oxygen or an in- 

 crease in the percentage of CO 2 in the blood will increase the activity 

 of the center. In asphyxia especially the center is extremely excit- 

 able, as shown by rise of the arterial tension. The subsidiary centers 

 in the spinal cord are influenced by corresponding conditions. 



Reflex Stimulation of the Vaso-motor Centers. The results 

 of experiment make it certain that the degree of vascular contraction 



