CONDITIONS INFLUENCING THE BLOOD-VESSELS. 675 



[If a rabbit be poisoned with curara, and the central end of the great auricular 

 nerve be stimulated, there is a double effect one local and the other general ; the 

 blood-vessels throughout the body, but especially in the splanchnic area, contract, 

 so that there is a general rise of the blood-pressure, while the blood-vessels of the 

 ear are dilated. If the central end of the tibial nerve be stimulated, there is a rise 

 of the general blood-pressure, but a local dilatation of the saphena artery in the 

 limb of that side (Loven). Again, the temperature of one hand and the condition 

 of its blood-vessels influence that of the other. If one hand be dipped in cold 

 water, the temperature of the other hand falls. Thus, pressor and depressor effects 

 may be obtained from the same nerve. The vaso-motor centre, therefore, primarily 

 regulates the condition of the blood-vessels, but through them it obtains its 

 importance by regulating and controlling the blood supply according to the needs 

 of an organ.] 



The central artery of a rabbit's ear contracts regularly and rhythmically 3 to 5 times per 

 minute. Schiff observed that stimulation of sensory nerves caused a dilatation of the artery, 

 which was preceded by a slight temporary constriction. 



Depressor effects are produced in the area of an artery on which direct pressure is made, as 

 occurs, for example, when the sphygmograph is applied for a long time the pulse-curves 

 become larger, and there are signs of diminished arterial tension ( 75). 



Khythmical Contraction of Arteries. In the intact body slow alternating contraction and 

 dilatation, without a uniform rhythm, have been observed in the arteries of the ear of the 

 rabbit, the membrane of a bat's wing, and the web of a frog's foot. This arrangement, observed 

 by Schiff, supplies more or less blood to the parts according to the action of external conditions. 

 It has been called a "periodic regulatory vascular movement." This movement may be use- 

 ful when a vessel is occluded, as after ligature, and may help to establish more rapidly the 

 collateral circulation. Stefani has shown that this occurs with more difficulty after section of 

 the nerves. 



Direct, local, applications may influence the lumen of the blood-vessels ; cold and moderate 

 electrical stimuli cause contraction ; while, conversely, heat and strong mechanical or 

 electrical stimuli cause dilatation, although with the last two there is usually a preliminary 

 constriction. 



Poisons. Almost all the digitalis group of substances cause constriction ; quinine and 

 salicin constrict the splenic vessels. The other febrifuges dilate the vessels {Thomson). 

 See p. 95. 



Effect on Temperature. The vaso-motor nerves influence the temperature, not 

 only of individual parts, but of the whole body. 



1. Local Effects. Section of a peripheral vaso-motor nerve, e.g., the cervical 

 sympathetic, is followed by dilatation of the blood-vessels of the parts supplied 

 by it (such as the ear of the rabbit), the intra-arterial pressure dilating the paralysed 

 walls of the vessels. Much arterial blood, therefore, passes into and causes 

 congestion and redness of the parts, or hyperemia, while, at the same time, the 

 temperature is increased. There is also increased transudation through the dilated 

 capillaries within the dilated areas ; the velocity of the blood-stream is of course 

 diminished, and the blood-pressure increased. The pulse is also felt more easily, 

 because the blood-vessels are dilated. Owing to the increase of the blood-stream, the 

 blood may flow from the veins almost arterial (bright red) in its characters, and the 

 pulse may even be propagated into the veins, so that the blood spouts from them 

 (CI. Bernard). Stimulation of the peripheral end of a vaso-motor nerve causes 

 the opposite results pallor, owing to contraction of the vessels, diminished 

 transudation, and fall of the temperature on the surface. The smaller arteries 

 may contract so much that their lumen is almost obliterated. Continued stimulation 

 ultimately exhausts the nerve, and causes at the same time the phenomena of 

 paralysis of the vascular wall. 



Secondary Eesults. The immediate results of paralysis of the vaso-motor nerves lead to 

 other effects ; the paralysis of the muscles of the blood-vessels must lead to congestion of the 

 blood in the part ; the blood moves more slowly, so that the parts in contact with the air cool 

 more easily, and hence the first stage of increase of the temperature may be followed by a fall 

 of the temperature. The ear of a rabbit with the sympathetic divided, after several weeks 



