. SPECIAL REFLEX CENTRES. 625 



the ordinary paths they have been educated to follow), and thus 

 convulsive movements are excited in many parts of the body. 



In like manner we can imagine that the unremitting activity 

 necessary to keep in check the impulses arriving from a constant 

 source of stimulation, such as intestinal worms, eventually fatigues 

 the active elements in this inhibitory mechanism, and then often 

 suddenly the force of the accumulated irritation, no longer re- 

 strained by the checking influence, rushes along the direct chan- 

 nels 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 j 1 ^ of a second. 

 The time required for the performance of a reflex act varies con- 

 siderably 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 



