THE VASOMOTOR CENTER AND NERVES. 763 



influence upon the bleeding artery. In this way is to be explained the phenomenon 

 familiar to surgeons that dangerous hemorrhage often ceases as soon as anemic 

 syncope occurs. In the frog, after ligation of the heart, all of the blood is finally 

 driven into the veins, likewise as a result of anemic irritation of the medulla. 

 In mammals the equalization of the blood-pressure in the arterial and the venous 

 system that follows exclusion of the heart takes place more slowly after destruction 

 than after preservation of the medulla. 



Among poisons, strychnin stimulates the center directly, even in curarized 

 dogs; and nicotin and calabar bean have the same effect. 



In animals in which the center is irritated electrically, it has been found that 

 single induction-shocks of moderate strength are effective only when two or three 

 shocks occur in a second. There is thus a summation of the effects of the indi- 

 vidual stimuli. The maximum vasoconstrictor effect, which can be recognized 

 from the maximum blood-pressure, is observed as a result of from ten to twelve 

 strong or from twenty to twenty-five moderately strong shocks in one second. 



The course of the vasomotor nerves is such that in part medullated and in part 

 non-medullated nerve-fibers, partly mixed with ganglion-cells, pass to the muscular 

 coats of the vessels. They pass from their center in part directly through the 

 tract of some of the cerebral nerves to their distribution; through the trigeminus 

 in part to the interior of the eye, through the hypoglossus to the tongue, through 

 fibers of the vagus to the heart and in limited number to the lungs and to the 

 intestines. All other vasomotor nerves descend in the lateral column of the spinal 

 cord (so that irritation of the lower extremity of the divided cord causes con- 

 striction of the vessels supplied from a lower level), and are connected within 

 the gray matter with centers of subordinate significance by means of contact. 

 They make their exit through the anterior roots of the spinal nerves, then pass 

 through the visceral branches into the ganglia of the sympathetic cord, where 

 the ganglion-cells are intercalated in the course of the individual fibers. In the 

 sympathetic cord they pass upward or downward and finally hence either to the 

 vascular plexuses or through other visceral branches again into the trunks of 

 spinal or cerebral nerves and from these to the respective vessels. 



In detail, the distribution in the cerebral region is as follows: The cervical 

 division of the sympathetic supplies in largest measure the head. In its area of 

 innervation the great auricular nerve in some animals also supplies a number of 

 vasomotors, which, in the rabbit, however, are derived from the inferior cervical 

 ganglion of the sympathetic. The cerebral vessels are supplied principally by the 

 sympathetic, irritation of which slows the blood-current in the small cerebral 

 arteries and increases the resistance in them; on the other hand dyspnea, as well 

 as administration of chloroform and amyl nitrite, causes acceleration of the blood- 

 current. The nerves reach the cerebral vessels not only through the cervical 

 sympathetic, but also through other tracts. The superior ganglion of the cervical 

 sympathetic supplies the thyroid gland. 



The upper extremities receive their vasomotor nerves through the anterior 

 roots of the dorsal nerves from the fourth to the tenth and thence through the 

 sympathetic cord to the first thoracic ganglion and from this through visceral 

 branches to the brachial plexus. The vasomotors for the skin of the trunk are 

 derived from the dorsal and lumbar nerves. The last three dorsal and the three 

 uppermost lumbar nerves contain the fibers for the lower extremity (in the dog) , 

 which first pass through the sixth and seventh lumbar and the first and second 

 sacral ganglia and then enter the trunks of the lumbar and sacral plexuses. 



The lungs are supplied (in addition to a number of fibers in the vagus) by 

 the first thoracic ganglion. According to Fr. Franck the sympathetic supplies 

 vasoconstrictors to the lesser circulation, arising from the second and third dorsal 

 nerves. They are stimulated reflexly through irritation of sensory nerves. The 

 activity of the vasomotors of the lesser circulation is relatively slight. In the frog 

 the vagus supplies the vasomotors of the lungs. 



The splanchnic is the most important of all of the vasomotor nerves, supplying 

 the abdominal viscera. Its vasoconstrictor fibers arise from the fifth dorsal nerve 

 and below. Irritation of the communicating branches between the eleventh dorsal 

 and the second lumbar nerve causes marked dilatation after primary contraction 

 of the vessels. Dilatation is caused also by irritation of the vagus. Asphyxia 

 causes contraction of all of the vessels of the entire intestine, the liver, and the 

 pancreas. Irritation of sensory nerves, for example the crural, causes reflex con- 

 traction of the vessels of the small intestine, the kidney, the spleen, the pancreas, 

 and dilatation of the vessels of the large intestine. Irritation of the centripetal 



