342 ANIMAL NATURE AS A WHOLE. [in. 



sion, whether arising from conceptions or other stimuli, con- 

 tinues its course onwards in the system of animal machines 

 from the place of its origin, whether that be in the brain, or in 

 a ganglion or plexus, or at the point of division of a nerve, or 

 any other point of the nerve-medulla, without putting any other 

 animal force into action, until it is conducted along other 

 branches or nerves (which duly receive it), either in a ganglion, 

 a plexus, or point of division, and thus excites animal actions 

 (124, 485) ; or until its own action excites external impressions 

 in the nerves, which again have their peculiar course and 

 operation (225). 



673. Those portions of the system of animal machines which 

 combine several natural connecting points of the animal forces, 

 are termed centres of the animal forces. A number of animal 

 machines are put into action, by means of these centres, when 

 external or internal impressions reach them. We will mention 

 the principal of these centres. The brain deserves the name in 

 a two-fold manner. Firstly, in its relation to the vital spirits, 

 of which it is the secreting organ, partly sending them to all 

 other animal machines, or communicating internal impressions 

 not caused by conceptions : partly receiving them back again 

 from the machines, or having external impressions commimi- 

 cated to it from the latter (11, 17, 18, 31). Now, although 

 many insentient animals have no proper brain, still all must 

 have animal machines performing this function, because there 

 can be no animal life without the intervention of the vital 

 spirits, and consequently without their secretion and circula- 

 tion. The ganglia and plexuses are probably, in this respect, 

 the analogues of the brain ; and in those animals, a mere section 

 of which has an independent existence, as polypes, or in those 

 in which the head and brain may be removed without injury 

 to life, since they are reproduced, as snails [vide Spallanzani), 

 either the whole nerve-medulla of the entire system of animal 

 machines, or a special portion in each limb, has the same func- 

 tion as ganglia and plexuses; so that animals of this class 

 possess several analogues of the brain (362, vide Haller's 

 'Physiology,' part iv, vol. x, sect, vii, § 36). Now, although 

 a brain of this kind may not be so constituted, as to be also 

 capable of the animal-sentient forces, as in sentient animals, 

 still it possesses so much of the structure of a brain, as to be 



