

358 NERVOUS SYSTEM DEVELOPEMENT. 



in number so as to form a body of considerable size, which is called 

 a ganglion. The position and mutual relations of these fibres and 

 ganglia may be best explained by reference to the mode of 

 developement of the cerebro-spinal axis in animals and in man. 



The centre of the nervous system in the lowest animals possessed 

 of a lengthened axis, presents itself in the form of a double cord. 

 A step higher in the animal scale, and knots or ganglia are 

 developed upon one extremity of this cord ; such is the most rudi- 

 mentary condition of the brain in the lowest forms of vertebrata. 

 In the lowest fishes the anterior extremity of the double cord dis- 

 plays a succession of five pairs of ganglia. The higher fishes and 

 amphibia appear to have a different disposition of these primitive 

 ganglia. The first two have become fused into a single ganglion, 

 and then follow only three pairs of symmetrical ganglia. But if the 

 larger pair be unfolded after being hardened in alcohol, it will then 

 be seen that the whole number of ganglia exists, but that four have 

 become concealed by a thin covering that has spread across them. 

 This condition of the brain carries us upwards in the animal 

 scale even to Mammalia ; e. g., in the dog or cat we find, first a 

 single ganglion, the cerebellum, then three pairs following each 

 other in succession ; and if we unfold the middle pair, we shall be 

 at once convinced that it is indeed composed of two pairs of pri- 

 mitive ganglia concealed by an additional developement. Again 

 it will be observed, that the primitive ganglia of opposite sides, at 

 first separate and disjoined, become connected by means of trans- 

 verse fibres 'of communication (commissures, commissura, a joining). 

 The office of these commissures is the association in function of 

 the two symmetrical portions. Hence we arrive at the general and 

 important conclusion, that the brain among the lower animals con- 

 sists of primitive cords, primitive ganglia upon those cords, and 

 commissures which connect the substances of the adjoining ganglia, 

 and associate their functions. 



In the developement of the cerebro-spinal axis in man, the earliest 

 indication of the spinal cord is presented under the form of a pair 

 of minute longitudinal filaments placed side by side. Upon these, 

 towards the anterior extremity, five pairs of minute swellings are 

 observed, not disposed in a straight line as in fishes, but curved 

 upon each other so as to correspond with the direction of the future 

 cranium. The posterior pair soon becomes cemented upon the middle 

 line ; forming a single ganglion ; the second pair also unite with each 

 other ; the third and fourth pairs, at first distinct, are speedily veiled 

 by a lateral developement, which arches backwards and conceals 

 them ; the anterior pairs, at first very small, decrease in size and 

 become almost lost in the increased developement of the preceding 

 pairs. 



We see here a chain of resemblances corresponding with the pro- 

 gressive developement observed in the lower animals ; the human 

 brain is passing through the phases of improving developement, 

 which distinguish the higher from the lower creatures : and we are 



