INNERVATION OF THE BLOOD-VESSELS OF DIFFERENT ORGANS 413 



region near the nucleus of the facial nerve and the superior olivary- 

 body. In the rabbit it possesses a length of 3 mm. and a breadth of 

 1-1.5 mm. A general, vasodilator center has not been definitely 

 located as yet, but it may be assumed to form either a part of the vaso- 

 constrictor center or to be situated in its immediate vicinity. 



Secondary centers controlling the caliber ^of the blood-vessels 

 are supposed to exist at different levels of the cord, as well as in the 

 sympathetic system, but the evidence upon which this statement is 

 based is not very conclusive. Thus, it has been found that the 

 tonicity of the blood-vessels is retained in a measure even after they 

 have been separated from the central nervous system and that their 

 tonus frequently reappears very soon after the division of the cervical 

 segment of the spinal cord. 



The. Activity of the Vasomotor Center. — Under normal conditions, 

 the activity of the vasomotor center is dependent upon an influx of 

 extraneous impulses. The sum total of these determines the tonicity 

 and the dynamic state of the vascular system. Its function may be 

 continued for some time after all these different afferent impulses 

 have been shut off, but naturally, a continued absence of these stimuli 

 always tends toward retrogression and functional uselessness. But, 

 besides these "external" impulses which are conducted to it by way of 

 many different centripetal nerves, the constituents of the vasomotor 

 center are also influenced by ''internal" stimuli, such as arise in 

 consequence of changes in its blood supply or variations in the gas 

 content of the blood. 



Thus, if the carbon dioxid of the blood is increased, as can readily 

 be done in a curarized animal by discontinuing the artificial respiration, 

 the general blood pressure will be seen to rise gradually until it attains 

 a height much above normal. The pressure usually remains at this 

 level for a considerable period of time, but declines subsequently on 

 account of the increasing diastolic tendency of the heart. This rise 

 is occasioned by a general constriction of the blood-vessels which is 

 dependent upon the direct excitation of the vasoconstrictor center by 

 the carbon dioxid. Eventually, however, the contractions of the 

 heart lose their force, because the continuous supply of blood poor in 

 oxygen, reduces its strength so that it is no longer able to act against 

 the high peripheral resistance occasioned by the vasoconstriction. 

 The blood pressure then falls in proportion to the diminution in the 

 energy of the heart and obviously, this fall must result in spite of the 

 fact that the blood-vessels remain in the constricted condition. If 

 the dyspneic or asphyctic condition of the blood is now lessened by 

 again instituting artificial respiration, the heart usually regains its 

 vigor within a short time. This change is clearly betrayed by a rise 

 in the blood pressure. above normal. Presently, however, the relaxa- 

 tion of the blood-vessels following upon the restitution of the vigor of 

 the cardiac contractions permits the pressure to become normal again. 

 Should the dyspnea and asphyxia be continued, a narcotic and para- 



