676 



FUNCTIONS OF THE SPINAL CORD 



[CH. XLVII. 



The reaction time in response to various kinds of stimuli, sound, light, pain, 

 etc., varies a good deal; the condition of the subject of the experiment is also an 



FIG. 491. The Dilemma. 



important factor. This, however, is really a practical branch of psychology, and 

 has recently been much worked at by students of that science (see also p. 204). 



Spinal Visceral Reflexes. 



The spinal grey matter contains centres which regulate the 

 operation of certain involuntary muscles. Some of these centres 

 are: 



The cilio-spinal centre controls the dilatation of the pupil ; it is 

 situated in the lower cervical region, reaching as far down as the 

 origin of the first to the third thoracic nerve. 



Subsidiary vaso-motor centres. The principal vaso-motor centre 

 is situated in the bulb, and subsidiary centres are scattered through 

 the spinal grey matter (see p. 299). 



The same is probably true for all the muscular viscera, but 

 particular study has been directed to those in the pelvis, and centres 

 for micturition, defcecation, erection, and parturition are contained in 

 the lumbo-sacral region of the cord. If the spinal cord is cut through 

 above the situation of these centres, the result is in general terms 

 that any influence of the higher (voluntary) centres over these 

 actions is no longer possible. The actions in question are then 

 simply reflex ones occurring unconsciously at certain intervals, and 

 set in movement by the peripheral stimulus (fulness of bladder, or of 

 rectum, etc.). If the portion of the cord where these centres are 

 placed is entirely destroyed, the result is paralysis of the muscles 

 concerned, though in certain cases, even after such a severe injury, 

 some amount of recovery has been noticed, which must be attributed 

 to the peripheral ganglia being able to play the part of reflex centres. 



The viscera are supplied not only by efferent (motor and inhibi- 

 tory) nerves, but are connected to the central nervous system by certain 

 afferent channels, namely (1) the vagus ; (2) the spinal roots from 

 the thoracic and first two lumbar nerves ; and (3) the second, third, 

 and fourth sacral roots. Under normal circumstances, the afferent 



