764 THE VASOMOTOR CENTER AND NERVES. 



fibers of the vagus causes dilatation of the vessels of the intestine, the pancreas 

 and the kidney. The vasomotors of the liver are discussed on p. 313, those of 

 the kidneys on p. 514, and those of the spleen on p. 195. The vasomotors are 

 all medullated from their origin to the sympathetic cord. 



In general the vessels of the skin of the trunk and the extremities are inner- 

 vated by those nerves that supply the same parts also with other, for example 

 sensory, fibers. 



The various vascular areas respond differently with respect to the intensity 

 of the action of the vasomotors. These affect in greatest degree the vessels of the 

 peripheral portions of the body, for example the toes, the fingers, the ears, less 

 markedly the central areas, for example the lesser circulation. 



Reflex Stimulation of the Center. The most varied centripetal nerves 

 contain fibers, irritation of which has an influence on the vasomotor 

 center. Irritation of some of these nerves causes stimulation of the 

 center and, thus, increased contraction of the arteries, together with 

 increased blood-pressure. These are designated press or fibers. On 

 the other hand, there are fibers irritation of which causes reflex dimi- 

 nution in the irritability of the vasomotor center. The result is, there- 

 fore, the opposite of the former. The nerves act really as inhibitory 

 nerves of the center and are designated depressor fibers. 



Pressor fibers have already been pointed out as present in the superior 

 and inferior laryngeal nerves, also in the trigeminus, the direct irritation 

 of which has a pressor effect, which occurs also as a result of insufflation 

 of irritating vapors into the nose. Aubert and Roever discovered pressor 

 fibers in the cervical sympathetic. S. Mayer and Pribram observed 

 that mechanical irritation of the stomach, particularly of the serosa, has 

 a pressor effect. Irritation of any sensory nerve is said to cause first 

 a pressor effect. 



Thus O. Naumann observed after feeble electrical irritation of the skin at 

 first a pressor effect, namely contraction of the mesenteric vessels, the lungs and 

 the web, with simultaneous excitation of the heart's action and acceleration of 

 the circulation (in the frog). Strong irritation, however, had the opposite or 

 depressor effect, with simultaneous reduction in the heart's action. Griitzner and 

 Heidenhain observed a pressor effect from contact with the skin, while manipu- 

 lations causing severe pain were ineffective. Reflex alteration in the lumen of the 

 vessels and in the activity of the heart can be induced also through the cutaneous 

 application of heat and cold. Schuller observed contractions of the vessels of the 

 pia (in rabbits) after pinching of the skin, likewise after warm baths or the ap- 

 plication of warm compresses, while cold baths or compresses caused dilatation 

 of the vessels. Schuller interprets these phenomena in part as pressor and de- 

 pressor effects, although he considers the principal cause to consist in the con- 

 traction of the cutaneous vessels in consequence of the cold, resulting in increase 

 of the blood-pressure and therefore in dilatation of the vessels of the pia. Heat 

 naturally has the opposite effect. 



In man most forms of irritation of the sensory nerves, such as slight cutaneous 

 irritation, tickling (also disagreeable odors, a bitter or a sour taste, optical or 

 auditory irritation), cause reduction of temperature at the point of application, 

 and diminution in the volume of the affected extremity, at times also increase 

 in the general blood-pressure and alteration in the action of the heart. The opposite 

 effects are induced by painful impressions, as well as by the action of heat (also 

 agreeable odors and a sweet taste) . The forms of irritation first mentioned cause 

 at the same time dilatation of the cerebral vessels and increase in the amount 

 of blood in the skull, while the latter bring about the opposite results. The time 

 for the reflex is between three and five seconds. 



Depressor fibers, irritation of which lowers the activity of the vaso- 

 motor center, are present in many nerves. The depressor nerve of the 

 vagus has already received special mention. The trunk of the vagus 

 below the latter contains depressor fibers, and so also do its pulmonary 



