232 CIRCULATION OF THE BLOOD 



more rapidly through the organ and does not have time to undergo the changes 

 which it otherwise would in the capillaries; the color of the venous blood is 

 therefore lighter, and its properties are similar to those of arterial blood. 



If now the end of the cervical sympathetic toward the head be stimulated, 

 the arteries are constricted with powerful stimulation, so much so that their 

 lumen disappears; the venous blood flows slowly and has a dark color; the 

 blood flows but feebly from a fresh cut, and the temperature of the ear falls. 



Since section of the cervical sympathetic causes a vasodilation of the ear, 

 and its stimulation constricts the blood vessels in the same ear, it follows that 

 this nerve must contain fibers which preside over contraction of the muscular 

 coat in these vessels i. e., which are vasoconstrictor fibers for the ear. It fol- 

 lows moreover that these nerves must be under tonic i. e., continuous stimu- 

 lation from the central nervous system. 



Now we have nerves running to all, or at least to most, of the arterial 

 regions of the body, which have the same properties as those just described. 

 The vascular tonus maintained by their constant stimulation is of the utmost 

 importance. For should all the vessels for any reason be completely relaxed, 

 there would collect in them, especially in the veins, so great a quantity of 

 blood that the volume flowing back to the heart would not be sufficient to 

 maintain the necessary supply; the blood pressure would fall to a low level 

 and, although the heart might continue to act for a time, it would be unable 

 to accomplish anything. All of which means that the total quantity of blood 

 in the body is sufficient to fill the blood vessels to the proper extent, only when 

 they are partially constricted. 



The vasoconstrictors are given off from the central nervous system in the 

 anterior nerve roots, and are distributed to the sympathetic paths throughout 

 the whole body. The following results have been obtained so far with regard 

 to their course: 



Most of the vasoconstrictor nerves pass out from the thoracic portion of the 

 spinal cord. The nerves running to the head arise from the first to the fifth tho- 

 racic nerves, pass over into the cervical sympathetic and are distributed to the 

 different parts of the head. This is attested by the fact that stimulation of the 

 cervical sympathetic causes vasoconstriction in all the organs of the head. With 

 regard to the brain, however, results are less positive. While some authors assert 

 that they have found vasoconstrictor nerves for the brain in the cervical sympa- 

 thetic, others have come to the conclusion that although nerve fibers have been 

 demonstrated anatomically for the blood vessels of the brain, in general the blood 

 supply to this organ is not regulated by means of vasomotor nerves, but by 

 alterations in the supply to other organs of the body. 



With respect to the further course of these nerves to the head, our informa- 

 tion is very incomplete. According to some they pass over into the sympathetic 

 plexuses surrounding the blood vessels, according to others they unite with the 

 cranial nerves. The latter has been demonstrated for the tongue at least, since 

 its vasoconstrictor nerves run for the most part in the hypoglossal. 



The vasoconstrictor nerves of the anterior extremities pass out from the 

 spinal cord in the third to the tenth thoracic nerves, those of the posterior ex- 

 tremities in the eleventh thoracic to the third lumbar nerves. It is stated also 

 that the vasoconstrictor nerves of the toes are contained in the sixth lumbar to 

 the first sacral nerves. 



