THE CIRCULATION OF THE BLOOD. 373 



constrictor nerves to the blood-vessels. The causes of this activity or tonicity 

 have been difficult to formulate. In how far the activity of the center is 

 maintained by the chemic character of the blood and lymph by which it is 

 surrounded and in how far by a continuous inflow of nerve impulses 

 transmitted from all regions of the body is not readily determinable. The 

 following facts will show that 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 vascular 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 attended by a 

 fall in blood-pressure indicating a decrease in the activity of the center. A 

 diminution in the percentage of oxygen or an increase in the percentage 

 of CO 2 in the blood will increase the activity of the center. In asphyxia 

 especially, the center is extremely excitable, as shown by a rise of the arterial 

 pressure. 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 maintained 

 by the vaso-constrictor centers can be increased or decreased by nerve im- 

 pulses reflected to the cord and medulla from the periphery or from 

 the brain. The effect may be general, or local and confined to the area 

 from which the impulses arise. The following experiments may be cited 

 as illustrations: 



Stimulation of the central end of a divided posterior root of a spinal nerve 

 gives rise to increased vascular contraction, as shown by the rise of blood- 

 pressure. Stimulation of the central end of the divided sciatic will give rise 

 to opposite results, according to the strength of the stimulus, weak stimuli 

 producing dilatation, strong stimuli producing contraction of the vessels. 

 Stimulation of the central end of the divided vagus gives rise to dilatation of 

 the vessels of the lips, cheeks, and nasal and palatal mucous membranes. 

 Stimulation of the tongue is followed by dilatation of the vessels of the submax- 

 illary gland. Stimulation of certain branches of the vagus nerve is followed 

 by a passive dilatation of blood-vessels and a marked fall of pressure. 



A satisfactory explanation of these different results is, however, wanting. 

 By some investigators it is believed that the usual variations in the arteriole 

 contraction are the outcome of corresponding variations in the activity of 

 the general vaso-constrictor center, the result of nerve impulses coming 

 through afferent nerves. 



The preceding statements as to the effects on the degree of vascular con- 

 traction, and hence on the blood-pressure which follow stimulation of differ- 

 ent afferent nerves, has lead to the assumption that there are in most afferent 

 nerves two classes of nerve-fibers, though perhaps in varying proportions, one 

 of which when in activity augments, the other of which when in activity inhibits 

 the activity of the vaso-constrictor center. The former class is generally 

 termed pressor or excitator, the latter depressor or inhibitor fibers. 



