238 THE CIRCULATION OF THE BLOOD 



some time, but the explanation that has usually been given has been that 

 it is due to a direct effect of oxygen want on the center. 



The sensitivity of the medullary center towards the hydrogen-ion is 

 many times greater than that of the subsidiary centers in the spinal 

 cord. If an animal is kept alive by artificial respiration for some time 

 after cutting the cervical spinal cord, the subsidiary vasomotor centers 

 will, as "\ve have seen, gradually acquire a tonic action, and the lowered 

 blood pressure will gradually rise again. If, when this has been attained, 

 the animal is made to breathe an atmosphere rich in carbon dioxide, a sud- 

 den rise in blood pressure will occur, but to produce it a very much 

 greater percentage of this gas must be inspired than when the pathway 

 between the chief and subsidiary centers is intact. Whereas 5 per cent 

 carbon dioxide is sufficient to cause a rise of pressure in an animal hav- 

 ing its chief vasomotor center, it takes 25 per cent and upward to pro- 

 duce a like effect on a spinal animal; and similarly, although 2 c.c. of 

 N/15 lactic acid will stimulate the chief vasomotor center, it takes 5 c.c. 

 of N/2 to excite the spinal-cord centers. 



The Nerve Control 



However important hormones may be in maintaining a tonic stimula- 

 tion of the center, the more sudden changes in activity are mainly 

 brought about by afferent nerve impulses. The afferent impulses are 

 of two classes: (1) those causing a rise in blood pressure, called 

 pressor, and (2) those causing a fall in blood pressure, called depressor. 

 The effect produced on the arterial blood pressure by stimulation of 

 either pressor or depressor fibers is usually more or less evanescent, 

 especially in the case of the depressor fibers; and when the change fol- 

 lowing stimulation of the nerve passes off, the blood pressure always 

 returns to its former level. This indicates that the afferent impulses do 

 not affect the tonic control which the vasomotor center exereises on the 

 blood vessels. It has, therefore, been assumed by Porter 10 that there are 

 really two kinds of A-asomotor centers: one concerned merely in the 

 bringing about of temporary reflex changes, the other concerned in the 

 maintenance of the vascular tone. It may be that the activities of the 

 former are primarily dependent upon afferent impulses, and the latter, 

 upon hormones. Justification for this view has been found in observa- 

 tions made on the effects of stimulation of pressor and depressor fibers 

 in animals under the influence of curare or alcohol. With the former 

 drug, stimulation of a nerve containing a preponderance of pressor or 

 depressor fibers produces double its usual effect, but the mean level of 

 the blood pressure apart from this effect remains unchanged. With the 

 latter drug (alcohol), on the other hand, the reflex response entirely 



