THE VASOMOTOR CENTER AND NERVES. 765 



branches (in the dog). The latter have a depressant effect also in con- 

 nection with marked expiratory pressure. In accordance with this fact 

 Hering showed that marked distention of the lungs (at a pressure of 50 

 mm. of mercury) caused lowering of the blood-pressure and acceleration 

 of the heart -beat. Irritation of sensory nerves, particularly if intense 

 and long continued, causes dilatation of the vessels in the areas innervated 

 by them. Also irritation of the muscle-nerves by pressure has a de- 

 pressant effect. According to Latschenberger and Deahna all sensory 

 nerves contain both pressor and depressor fibers. 



Schiff observed after irritation of sensory nerves the periodic contractions in 

 the ear of the rabbit, normally occurring from three to five times in a minute, 

 give way to dilatation, after a preceding contraction of short duration. Direct 

 pressure upon an artery within its distribution has a depressor effect, as can be 

 seen, for example, from the fact that after long-continued pressure of the sphygmo- 

 graph the pulse-tracing becomes larger and exhibits signs of lessened arterial 

 tension. 



In the intact body slowly alternating contraction and dilatation, 

 without uniform rhythm, are observed in the arterial branches (arteries 

 of the rabbit's ear, in the membrane of the bat's wing, the web of the 

 frog's foot). This movement, discovered by Schiff, is for the purpose 

 of supplying the organ in question at times with a larger, at other times 

 with a smaller, supply of blood, accordingly as its nutrition or external 

 influences demand. This phenomenon can appropriately be designated 

 periodic-regulatory vascular movement. It is probably responsible, in 

 case of occlusion of the vessels, for example after ligature, for the prompt 

 establishment of the collateral circulation. This occurs with distinctly 

 greater difficulty after division of the nerve. 



According to Bier the final cause resides in the cellular activity in the tissues 

 that have become anemic. After transitory anemia the small vessels of the skin 

 open widely for the reception of the arterial blood-current, while they close to 

 the venous current, and, moreover, independently of the central nervous system. 

 Nothnagel agrees with v. Recklinghausen that the increased velocity with which 

 the blood flows through the collateral branches of the obstructed vessel is the 

 factor that causes hypertrophy of the walls of the vessel and dilatation of the 

 lumen of the collateral branches. 



Perhaps the arteries are capable of another form of movement, namely the 

 pulsatory, which consists in active contraction after each pulsatory dilatation 

 of the vessel. It would, therefore, correspond with the registration of the de- 

 scending limb in the tracing. From what has been said as to the propagation- 

 velocity of the pulse-wave, this contraction must be propagated centrifugally in 

 a peristaltic manner with the same velocity as the pulse-waves. It should, how- 

 ever, be stated that, as yet, this form of movement has not been demonstrated 

 with certainty. 



The lumen of the vessel can be influenced directly by local applica- 

 tion, cold and moderate electrical stimulation causing contraction, and, 

 conversely, heat and strong mechanical or electrical stimulation, causing 

 dilatation, the latter two probably after transitory preceding contrac- 

 tion. 



Elevation of the temperature of the arm to 43 C. causes relaxation of the vessels, 

 reduction to between 10 and 20 C. contraction. Abrupt alterations in tempera- 

 ture always cause transitory contraction of the vessels (also those of the opposite 

 arm). Irritation by heat and cold is capable, in addition to its influence upon 

 the vessels themselves locally at the seat of irritation, also of affecting the lumen 

 of the vessels through reflex stimulation. Thus cold, for example, may cause 

 dilatation of the vessels. Electrical stimuli likewise exert their effects principally 



