CIRCULATION OF THE BLOOD. 555 



will become dilated. This, therefore, indicates that some portion of the 

 brain higher up than the cervical spinal cord is the centre from which 

 originate all the impulses which travel along the different vaso-motor 

 nerves to innervate the muscular fibres of the different arterioles of the 

 body. When the path of communication between anj^ vascular area and 

 the medulla oblongata is broken, the vessels of that part are deprived of 

 the nervous impulse coming from the medulla, they lose tone, their mus- 

 cular fibres relax, and their arteries dilate. When the entire body is cut 

 off from the medulla all the vessels, therefore, dilate. It has been stated 

 that stimulation of one of the vaso-motor nerves — such an example being 

 the sympathetic, the sciatic, or splanchnic — leads to contraction of the 

 arterioles in the corresponding vascular area. In other words, the 

 electric stimulation has been added to that coming constantly from the 

 medulla, and has therefore increased the contraction of the muscular 

 fibres of the arteries. So, also, if the spinal cord be itself stimulated, 

 the blood-vessels in all the parts below become still further contracted, 

 while by directly stimulating the medulla oblongata all the blood-vessels 

 of the body contract. These facts indicate that the normal degree of 

 constriction of the arterioles of the body, or what is termed vascular 

 tonus, is maintained by a series of impulses constantly coming from 

 a collection of cells in the medulla oblongata, called the vaso-motor 

 centre. These impulses reach the arteries after passing through the 

 lateral columns of the cord, during which passage they make connection 

 with the subordinate vaso-motor centres of the cord, either through the 

 anterior spinal roots directly or by passing through the rami commu- 

 nicantes to the sympathetic. 



The vaso-motor centre has been located in the floor of the fourth 

 ventricle, its lower limit being a horizontal line about four or five milli- 

 meters above the point of the calamus scriptorius and the upper limit 

 about four millimeters higher up, or one or two millimeters below the 

 corpora quadrigemina. The vaso-motor centre, as already stated, may 

 be directly stimulated, and so lead to increased vascular contraction 

 throughout the entire body. Increased venosity of the blood and vari- 

 ous poisons, such as strychnine, likewise directly stimulate the vaso- 

 motor centre, and so cause increased blood pressure ; so, after death, the 

 venous character of the blood, through stimulation of the vaso-motor 

 centre, leads to firm contraction of the arteries and a consequent empty- 

 ing of these vessels into the veins. It may also be reflexly stimulated. 

 Irritation of any sensory nerve will reflexly act on the vaso-motor centre 

 and lead to arterial contraction, especially of the vessels of the abdomen. 

 On the other hand, the vaso-motor centre may be inhibited. 



If the small nervous filament which is formed in the rabbit by the 

 union of branches from the pneumogastric and superior laryngeal nerves 



