3 88 TEXT-BOOK OF PHYSIOLOGY 



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

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

 the spinal cord are influenced by corresponding conditions. 



Reflex Stimulation of the Vase-reflex Center. The results of experi- 

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

 or irihibited in its activity, and hence followed by increased or decreased 

 vascular contraction, by nerve impulses transmitted to it from the brain, or 

 from the periphery through afferent nerves. The effects may be local and 

 confined to the area in which the stimulus is acting or they may be general. 

 The following experiments may be cited in illustration: 



Psychic states of an affective or emotional character influence the ac- 

 tivity of this center, doubtless as a result of the arrival of nerve impulses 

 from the cortex of the cerebrum. Thus it is well known that fear causes a 

 contraction of the blood-vessels of the head and face and that shame causes 

 a dilatation of the same vessels. With the cessation or the disappearance 

 of the emotional state, the blood-vessels return to their former degree of 

 contraction. 



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. 



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. 



Theories of the Mode of Action of the Vaso-motor Centers. A 

 satisfactory explanation of the manner in which peripheral stimuli influence 

 the activity of the vaso-motor centers and thus bring about the opposite 

 results mentioned in the foregoing paragraphs is difficult to present. Several 

 interpretations have been offered, viz. : 



1. The peripheral stimuli according to their qualities act on either the ter- 



minals of the pressor or the depressor fibers; according as they do, will 

 the vaso-constrictor center be augmented or inhibited in its activity 

 and followed by either an increase or decrease in the degree of the 

 previous vascular contraction. 



2. The peripheral stimuli act simultaneously on the terminals of both 



pressor and depressor fibers, and hence the vaso-constrictor center is 

 acted on simultaneously by two antagonistic influences. The resulting 

 effect on the blood-vessels, viz., increased or decreased contraction will 

 be the resultant of the action of these two influences on the vaso-con- 

 strictor center. If the stimuli act preponderantly on the pressor fibers, 



