626 MANUAL OF PHYSIOLOGY. 



arriving from the urethra, near the neck of the bladder. It then 

 sets the detrusor muscle in action, while the sphincter is relaxed 

 by voluntary inhibition. 



3. The ejaculation of the semen may also be said to be accom- 

 plished by a special spinal centre, capable of controlling certain 

 special movements, in which involuntary muscles play an im- 

 portant part. 



4. In parturition a number of motions are called into play (as 

 well as the uterine contraction) which are so regularly coordinated 

 as to entitle us to suppose that they are arranged by a special 

 centre in the spinal cord. 



5. The act of defecation is accomplished by means of a spinal 

 centre also. The action of this centre might (like that presiding 

 over the urinary bladder) be divided into two parts retention 

 and evacuation in which volition and intestinal peristalsis play 

 a very important part. 



AUTOMATISM. 



Besides being excited to action by impulses coming from the 

 brain volition and from the surface reflection the groups of 

 cells in the spinal cord may act without any obvious incoming im- 

 pulse ; that is to say, some of the cells are capable of spontaneous 

 activity. Such groups of nerve-cells are commonly called auto- 

 matic centres; the more important of those found in mammalia 

 may be classified as follows : 



1. Vaso-motor centres: Though the central point from which 

 the contraction of the bloodvessels is controlled is situated in the 

 medulla, there is no doubt that even in mammalia centres are dis- 

 tributed throughout the gray matter of the spinal marrow, which 

 are capable of keeping up the arterial tone in the regions to which 

 they correspond. As evidence of this may be mentioned the fact 

 that the dilatation of the arteries, which follows the severance of 

 the lumbar part of the cord from the medulla, only lasts a few 

 days, after which the vessels again contract in the usual tonic 

 manner. The arterial tonus only disappears completely and 

 permanently when the spinal cord is destroyed. Thus, it would 

 appear although habitually the vessels of all the body are regu- 



