372 . TEXT-BOOK OF PHYSIOLOGY. 



that they respond to nerve impulses transmitted to them from different regions 

 of the body, as shown by the contraction of blood-vessels. This is especially 

 true of lower animals such as the frog and it may possibly be true of 

 mammals. Though it is probable that the spinal vaso-constrictor cells 

 possess a certain degree of tonicity, nevertheless they are subordinate in 

 their activity to and dominated by a group of nerve cells in the upper part 

 of the floor of the fourth ventricle and termed for this reason the medullary 

 bulbar vaSo-constrictor center. 



Though the blood-pressure falls to a very low level after the separa- 

 tion of the medulla from the spinal cord, the animal, if properly cared for, 

 may survive the operation and live for a considerable time. Under these 

 circumstances the arteries gradually recover their former degree of con- 

 traction. This is accepted as evidence that the nerve cells in the spinal 

 cord have acquired an independent activity, or developed an activity that 

 had hitherto been dormant. After this, these nerve centers can be excited 

 to activity by nerve impulses transmitted from the periphery. 



The Medullary or Bulbar Vaso-Constrictor Center. The existence of 

 such a dominating center has been determined experimentally: thus if a 

 definite region of the medulla oblongata is punctured or in anyway destroyed 

 there is an immediate dilatation of the blood-vessels throughout the body 

 and a fall of blood-pressure below one-half or one-third of the normal value. 

 This region has a width of one and a half millimeters and extends longitudin- 

 ally for a distance of four or five millimeters, terminating at a point four 

 millimeters above the tip of the calamus scriptorius. Because of the effects 

 that follow the destruction of this area the anatomic existence of a general 

 vaso-constrictor center has been assumed. 



A transection of the medulla above the upper limit of this area is without 

 effect on the blood-pressure. A similar section below it, however, is at once 

 followed by vascular dilatation, a loss of vascular tone, and a general fall of 

 blood-pressure. Subsequent stimulation of the peripheral end of the divided 

 medulla, the animal being curarized and artificial respiration maintained, will 

 give rise to a marked contraction of the blood-vessels and a rise of blood- 

 pressure up to and far beyond the normal value. 



If the experimental lesion is limited to the area mentioned in the foregoing 

 paragraph, the vascular dilatation also passes away after a time, the blood- 

 vessels regain their normal tone, and the pressure again rises. These and 

 the foregoing facts indicate that there is in the gray matter beneath the floor 

 of the fourth ventricle a restricted area composed of nerve-cells, which main- 

 tains through efferent nerve-fibers the tonus of the blood-vessels by virtue 

 of its dominating influence over the vaso-motor centers in the cord, and 

 which is therefore to be regarded as the general vase-motor (constrictor) 

 center. The vaso-motor centers throughout the cord are to be regarded as 

 subsidiary centers. The nerve-fibers which transmit the regulative nerve 

 impu ses from the general to the subsidiary centers are to be found in the 

 lateral columns of the spinal cord. 



The Tonic Activity of the General Vaso-constrictor Center- 

 Since the blood-vessels maintain a more or less constant tone, it is assumed 

 that the vaso-motor center is in a state of continuous tonic activity or 

 tonus, and as a result continuously discharging nerve impulses through vaso- 



