The Nervous System. 279 



In the medulla is situated a cardio-accelerator and cardio- 

 inhibitory centre. The contraction of the heart is regulated 

 by nerves passing from a centre in the medulla ; one nerve, 

 the vagus, passes down the neck, the function of which is 

 to inhibit or restrain the action of the heart ; it is brought 

 into action either by automatic impulses originating in the 

 medulla through the blood circulating in it, or reflexly by 

 impressions from without, and it is always in operation. 

 Accelerating influences, from a centre in the medulla, pass 

 to the heart through the sympathetic, the fibres for which 

 issue from the spinal cord in the lower cervical or anterior 

 dorsal region, and pass to the inferior cervical ganglion and 

 then to the heart ; the impulses which accelerate the heart 

 are not in constant action (see p. 59). 



A vaso-motor or vessel-moving centre is situated in the 

 floor of the fourth ventricle ; by some it is considered to 

 •consist of two parts, a vaso-dilator and a vaso- constrictor 

 •centre. The vaso-constrictor centre is constantly keeping 

 the vessels in the condition known as ' tone ' ; if the 

 •centre be irritated, the vessels contract and the blood 

 pressure rises. Under the influence of the vaso-dilator 

 nerves, the vessels dilate and the blood pressure falls 

 {see p. 72). 



The vaso-motor centre is influenced not only through the 

 cord from below, but directly by the brain from above ; it 

 is also stimulated by the character of the blood circulating 

 in the medulla, and under all circumstances it exercises a 

 control over the vaso-motor centres in the cord. 



Diabetes, after puncturing the floor of the fourth 

 ventricle, is said to be explained by injury to the so-called 

 dilator centre, allowing the vessels of the liver to dilate 

 (see p. 1G7). The cause has, however, not as yet been 

 clearly made out. 



The sweat centre is stated to govern the sweat centres in 

 the spinal cord, and to produce sweating of the head ; it 

 can be excited reflexly or automatically. 



The medulla has no sensation ; it can originate no volun- 

 tary impulse ; it forms a pathway to the brain for the 



