Chap., Ill] 



OF MOTRICITY. 



889 



The first case is observed in certain rudimentary protozoaries, 

 in which the nervous system is reduced to some scattered cells, 

 forming with a fine fibre a network scarcely perceptible. The 

 second is found normally and 

 frequently in a number of in- 

 vertebrates with a ganglionary 

 nervous system. In the verte- 

 brates it is the ordinary mode 

 of fiinctionment of the sympa- 

 thetic nervous network. In 

 efiect it is by virtue of uncon- 

 scious reflex acts, going on in 

 the sympathetic ganglions (Fig. 

 72), that the motory incita- 

 tions are transmitted to the 

 smooth fibres of the intestine, 

 of the stomach, of the bladder, 

 of the sanguineous vessels ; and, 

 in sum, almost all the move- 

 ments of the apparatus of the 

 vegetative life • in the verte- 

 brates are achieved by virtue 

 of reflex ganglionary actions. 



The cells of the spinal marrow (Fig. 73) do not seem, spite 

 of what has been pretended, endowed with conscious sensibility ; 

 nevertheless they are very active, and the spinal marrow must 

 be considered one of the most important reflex centres, especially 

 for the muscles of the life of relation. It holds in effect tfnder 

 its sway a very great number of muscles, which owe to it that 

 permanent half contraction which is called tonicity, and which is 

 the cause of the constant contraction of the sphincters, of the 

 antagonist muscles ; and so on. This tonus depends so much on- 

 the spinal marrow, that it is instantaneously abolished by the 

 destraction of the marrow, whence the nerves of the muscles 

 indicated dei-ive their motricity. 



FlQ. 72. 



Peripheric ganglion of a mammifeT ; Bchematic 

 outline : a, h, c, three nerves proceeding 

 from the ganglion ; d, multipolar ganglion- 

 ary cells ; e, unipolar cells ; /, apolar cells. 



