THE CIRCULATION OF THE BLOOD. 



the abdorminal viscera which apparently receive vaso-dilatator nerve impulses 

 are supplied by nerves having the foregoing origin and action is a subject for 

 further investigation. 



Physiologic Properties. The vasoconstrictors and the vaso-dilatators 

 differ somewhat in their physiologic properties, as shown by the results 

 of experiment. Thus, when a mixed nerve, i.e., one containing both classes 

 of fibers e.g., the sciatic is stimulated with frequently repeated induced 

 currents, the constrictor effect is the more pronounced, the dilatator effect 

 being wanting or prevented; when stimulated with slowly repeated induced 

 currents, the dilatator effect is the more pronounced. These different effects 

 are strikingly shown in Fig. 175, A and B. 





A B 



FIG. 175. PLETHYSMOGRAMS OF THE HIND- LEG OF THE CAT FOLLOWING STIMULATION OF 

 THE SCIATIC NERVE. In A the rate of stimulation was sixteen per second, in B one per second 

 for fifteen seconds. 



In the experiment of which these tracings are the result the leg of a cat was 

 enclosed in a plethysmograph and the variations in volume due to dilatation 

 or contraction of the vessels, following stimulation of the sciatic nerve, were 

 recorded by means of tambour and lever on a slowly revolving cylinder. In 

 A the fall of the curve indicates a diminution of volume, from contraction of 

 blood-vessels following a rate of stimulation of the sciatic nerve of 16 per 

 second for fifteen seconds. In B the rise of the curve indicates an increase 

 in volume from dilatation of the vessels following a rate of stimulation of i 

 per second for fifteen seconds (Bowditch and Warren). With different rates 

 of stimulation somewhat different results are obtained. 



After division of a mixed nerve the vaso-constrictors degenerate and lose 

 their influence over the blood-vessel in from four to five days, the vaso- 

 dilatators in from seven to ten days, as shown by the response to electrical 

 stimulation. 



When a nerve is cooled, the vaso-constrictors lose their irritability before 

 the vaso-dilatators. 



Vaso-motor Constrictor Centers. The nerve-cells thoughout the 

 spinal cord from which the vaso-constrictor nerves take their origin may be 

 regarded as nerve-centers which through their related nerve-fibers cause a 

 varying degree of contraction of the arteriole muscle. In how far these 

 centers are independent in their activity it is difficult to state. From the 

 results of experiments that have been made with a view of isolating these 

 centers, such as division of the cord at different levels, it is fairly well proven 



