236 THE CIRCULATION OF THE BLOOD 



an increase in blood flowing into the vessels because of increased heart 

 action or to a local vasodilatation; and vice versa, when shrinkage oc- 

 curs. We can not tell from the volume tracing itself which of these 

 changes is really responsible for the observed alteration, but we can do 

 so by simultaneously observing the mean arterial blood pressure. If this 

 falls when the volume decreases, it means that the volume of blood flow- 

 ing to the part must have become diminished. If, on the other hand, the 

 blood pressure remains constant or rises while the volume decreases, it 

 means that the blood vessels have locally constricted. 



Methods for the Detection of Vasomotor Fibers in Nerve Trunks 



If we wish to find out through which nerve trunks a given vascular 

 area is supplied with vasoconstrictor or vasodilator impulses, we should 

 proceed by the use of one of the above described methods to observe the 

 effect produced on the vessels by cutting the nerve and then by stimu- 

 lating the peripheral end of the cut nerve. As a result of such observa- 

 tions it has been found that the vasomotor fibers are frequently dis- 

 tributed so that those having a vasoconstricting action are collected 

 mainly in one nerve trunk' and those having a dilating action in another; 

 in some nerve trunks, however, the relative numbers of the opposing 

 fibers are about equal. Nerves containing a great preponderance of vaso- 

 constrictor fibers are the great splanchnic and the cervical sympathetic; 

 and those containing a great preponderance of vasodilator are the chorda 

 tympani nerve to the submaxillary gland and the nervi erigentes to the 

 external genitalia. 



It must be clearly understood that, although one kind of vasomotor 

 fiber may preponderate in one of these nerves, yet the opposite kind is 

 also present. In the cervical sympathetic, for example, some vasodila- 

 tor fibers extending to the blood vessels of the mucous membrane of the 

 nose and cheeks can readily be demonstrated, as shown by the flushing 

 of these parts when the peripheral end of the nerve is stimulated; and 

 similarly, even in the great splanchnic nerve itself, vasodilator fibers 

 supplying the suprarenal capsule can quite readily be made out. When 

 the vasoconstrictor fibers greatly preponderate over the vasodilator, the 

 effect of the latter may be demonstrated by taking advantage of the fact 

 that ergotoxine paralyzes the vasoconstrictor but not the vasodilator 

 fibers, so that after its administration stimulation of the great splanch- 

 nic nerve gives rise to a vasodilatation instead of a vasoconstriction. 

 The presence of vasoconstrictor fibers in the so-called vasodilator nerves 

 (chorda tympani and nervi erigentes) has not however, been demon- 

 strated. 



A good example of a nerve trunk containing about an equal admix- 



