SPECIAL REFLEX CENTRES. 625 



the active elements in this inhibitory mechanism, and then- 

 often suddenly the force of the accumulated irritation, no longer 

 restrained by the checking influence, rushes along the direct 

 channels to all parts of the cord, and simultaneously exciting 

 them brings many discordant muscles into spasmodic action. 



The reflection of an impulse from a sensory nerve, through the 

 cells of the spinal cord to a motor nerve, occupies a measurable 

 length of time, which has been estimated at about ^ of a second. 

 The time required for the performance of a reflex act varies 

 considerably in the same individual under different conditions ; 

 of these, high temperature and intense stimulation shorten the 

 time, and fatigue or cold lengthen it. 



SPECIAL REFLEX CENTRES. 



Many of the groups of nerve cells in the cord are employed in 

 executing definite familiar acts essential to the animal economy 

 and more or less independent of the will. Many of these acts 

 are very complex, and require the coordinated action of certain 

 sets of muscles and the inactivity of others. Such groups of 

 nerve cells have been called special centres, and many of them 

 have already been referred to in the preceding chapters, where a 

 fuller consideration of them may be found. The more important 

 are: 



1. A centre for securing the retention of the urine by the tonic 

 contraction of the sphincter muscle of the bladder. This group 

 of nerve cells is probably kept in action by impulses arriving 

 from the bladder by the afferent nerves, passing from its walls to 

 the spinal cord. The more distended the bladder becomes, the 

 more powerful the stimulus sent to the cord, and, therefore, the 

 more firmly the sphincter is made to contract. 



2. Nearly related to the former is the centre which presides 

 over the evacuation of the bladder. This is excited by impulses 

 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 



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