THE CONTROL OF THE CIRCULATION 243 



v 



subsidiary centers is intact. Whereas 5 per cent carbon dioxide is sufficient to cause 

 a rise of pressure in an animal having its chief vasomotor center, it takes 25 per 

 cent and upward to produce 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 exercises on the 

 blood vessels. It has, therefore, been assumed by Porter 16 that there are 

 really two kinds of vasomotor 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 

 disappears, although it immediately reappears when the alcohol effect 

 has passed off, and there is no evidence of a change in tone. The tonic 

 and the reflex mechanisms of the vasomotor center can not therefore be 

 identical. 



At the present stage of our knowledge, it is only possible for us to 

 study the effect of stimulation of pressor and depressor fibers on the 

 vasoreflex center. Such fibers are contained in practically every sen- 

 sory nerve of the body, and it would appear that a fairly equal mixture 

 of both kinds of fiber exists in most of these nerves. 



Pressor and Depressor Impulses. Depressor impulses alone are present 

 in the cardiac depressor nerve. Sometimes as in the rabbit, this exists 

 as an independent nerve trunk, originating by two branches, one from 

 the superior laryngeal, the other from the vagus, and descending close to 



