188 PRINCIPLES OF PHYSIOLOGY 



the heart upwards. If we stimulate this 

 nerve, called the depressor, impulses pass 

 upwards which inhibit the centre in the 

 medulla, throwing it, as it were, out of action, 

 with the result that the arterioles dilate and 

 the blood pressure falls. Other nerves appar- 

 ently may affect this centre in an opposite 

 way, causing it to act more powerfully, and 

 therefore raising the blood pressure. These 

 are called pressor-nexves, in opposition to 

 the depressors. Most sensory nerves act as 

 pressor-nerves. Nerve centres are thus often 

 under the action of impulses having contrary 

 effects, while they are also influenced by the 

 quality of the blood circulating through 

 them. Inhibitory mechanisms play an import- 

 ant part in the nervous machine. Probably 

 the restraining powers of what we term the 

 will have a physiological basis of this nature. 

 106. The spinal cord may be regarded as a 

 series of segments combined together to form 

 one mass. Each segment has a pair of spinal 

 nerves, each connected with the central grey 

 matter by two roots. The anterior root 

 consists of motor fibres carrying nervous 



