762 THE VASOMOTOR CENTER AND NERVES. 



According to experiments of Strieker and Wagner section of both vagi in 

 the dog causes reduction in the number of heart-beats when the accelerator 

 fibers on both sides are divided. This circumstance would indicate a state of 

 tonic innervation of the latter. 



The center can be stimulated reflexly through irritation of the central stumps 

 of many sensory nerves. 



THE VASOMOTOR CENTER AND NERVES. 



The dominating center, which supplies all of the muscles of the arterial 

 system with motor fibers (vasomotors, vasoconstrictors), is situated in 

 the medulla oblongata at a point in part rich in large ganglia. It is 3 

 mm. long and i^ mm. wide in the rabbit and extends from the region 

 of the upper portion of the floor of the fourth ventricle to about 4 or 5 mm. 

 above the calamus scriptorius. Each half of the body has as its own 

 center, which is situated 2$ mm. from the middle line in that portion 

 of the medulla on each side that represents the prolongation of the lateral 

 columns of the spinal cord (lower portion of the superior olive). Irri- 

 tation of this central point causes contraction of all of the arteries and in 

 consequence increase in arterial blood-pressure, the veins and the heart 

 becoming distended. Paralysis of the center causes relaxation and 

 dilatation of all of the arteries, with enormous reduction in blood-pres- 

 sure. Under normal conditions the vasomotor center is in a state of 

 moderate tonic excitation. Like the cardiac inhibitory and the res- 

 piratory center, the vasomotor center can be stimulated directly and 

 reflexly. 



Direct Stimulation of the Center. In this connection the amount of 

 gases contained in the blood circulating in the medulla oblongata is of 

 paramount importance. In the state of apnea the center appears to be 

 in a condition of slightest excitation, as the blood-pressure is exceedingly 

 low. With the state of the blood present under normal conditions the 

 center is in a condition of moderate excitation. Fluctuations in the 

 irritation of the center accompany the respiratory movements (Traube- 

 Hering fluctuations), as can be seen from the simultaneous increase in 

 blood-pressure. When the blood presents marked venosity, in conse- 

 quence of asphyxia or insufflation of air rich in carbon dioxid, the center 

 is more actively stimulated, so that all of the arteries contract, with 

 marked increase in blood-pressure, and the venous system and the heart 

 are greatly distended with blood. Under such circumstances the veloc- 

 ity of the blood-current is increased. The same effect is produced by 

 sudden anemia of the medulla through ligation of both carotid and sub- 

 clavian arteries, and likewise by sudden stagnation of the blood in the 

 presence of venous hyperemia. 



The venosity of the blood that always develops after death causes quite con- 

 stantly active stimulation of the vasomotor center, as a result of which the arteries 

 are strongly contracted. As, in consequence of this, the blood is driven to the 

 capillaries and the veins, the state of emptiness of the arteries after death, which 

 was familiar to the ancients, is explained. 



Upon this circumstance is dependent also the fact, as Landois has found, that 

 hemorrhage from large wounds takes place much more freely when the vasomotor 

 center is preserved than when it has previously been destroyed (in the frog). 

 As emotional disturbances have a corresponding influence upon the vasomotor 

 center, their influence upon the control of hemorrhage is obvious Thus it has 

 been observed in hysterical individuals that a wound yields only one-third as 

 much blood as the same condition in a normal individual. If the hemorrhage is 

 considerable, the anemic irritation of the medulla may finally exert a constricting 



