THE CIRCULATION OF THE BLOOD 383 



vessels regain their normal tone, and the pressure again rises. These and 

 the foregoing facts indicate that there is in the gray matter beneath the floor 

 of the fourth ventricle a restricted area composed of nerve-cells, which main- 

 tains through efferent nerve-fibers the tonus of the blood-vessels by virtue 

 of its dominating influence over the vaso-motor centers in the cord, and 

 which is therefore to be regarded as the general vaso-motor (constrictor) 

 center. The vaso-motor centers throughout the cord are to be regarded as 

 subsidiary centers. The nerve-fibers which transmit the regulative nerve 

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

 lateral columns of the spinal cord. 



The Tonic Activity of the General Vaso-constrictor Center. 

 Since the blood-vessels maintain a more or less constant tone in the physio- 

 logic condition, it is assumed that the vaso-motor center is in a state of 

 continuous or tonic activity or tonus, and as a result continuously discharging 

 nerve impulses through vaso-constrictor nerves to the blood-vessels. The 

 causes of this activity or tonicity have been difficult to formulate. It may be 

 accepted, however, from the results of experimentation that the activity of 

 the center is maintained (i) by the chemic character of the blood and 

 lymph by which it is surrounded and (2) by a continuous inflow of nerve im- 

 pulses transmitted from all regions of the body, or to both. The follow- 

 ing facts will show that both factors are probably involved. 



The group of nerve cells which collectively constitute the vaso-motor 

 center has been shown by the experiments of Porter to consist of two groups 

 or centers, viz., a vaso-tonic center which maintains the vascular tonus, and a 

 vaso-reflex center which permit of various vaso-motor reflexes. After the 

 administration of curara the depressor and sciatic reflex changes in blood- 

 pressure are more than double, while the arterial tonus remains substantially 

 unchanged. Alcohol on the contrary diminishes or removes reflex activity, 

 but leaves the tonus unchanged. 



Central Stimulation of the Vaso-tonic Center. The general vaso- 

 motor (constrictor) center at least is markedly influenced b} 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 a rise of pressure, the result of increased vascular contraction. Restora- 

 tion of the blood-suply is followed by a return of the center to its normal 

 degree of activity. An increase of the percentage of carbon dioxid in the 

 blood or a decrease in the percentage of oxygen increases the activity^ the 

 center. In asphyxia, a condition in which there is a deficiency in the elimina- 

 tion of carbon dioxid and an insufficiency in the absorption of oxygen the 

 center becomes extremely excitable as shown by the rapid rise of the arterial 

 pressure. By reason of these and other facts it is assumed that the presence 

 and the normal pressure of CO 2 in and around the vaso-motor center is 

 the efficient stimulus to its activtiy. In compression of the carotid arteries 

 as well as in asphyxia, the CO 2 is not removed from the center sufficiently 

 rapidly and hence its pressure increases and unduly excites the center as 

 shown by the rapid rise of the blood-pressure. The subsidiary centers 

 the spinal cord are influenced by corresponding conditions. 



Reflex Stimulation of the Vaso-reflex Center. The results of expen 

 mentation render it certain that the vaso-reflex center may be augmented 



