400 NERVOUS SYSTEM. DEVELOPMENT. 



and mutual relations of these fibres and ganglia may be best explained 

 by reference to the mode of development 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 rudimentary condition of 

 the brain in the lowest forms of vertebrata. In the lowest fishes the 

 anterior extremity of the double cord displays 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 sym- 

 metrical ganglia. But if the larger pair be unfolded after being 

 hardened in alcohol, it will then be seen that the whole number of 

 ganglia exist, 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 composed of two pairs of 

 primitive ganglia concealed by an additional development. Again it 

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

 separate and disjoined, become connected by means of transverse fibres 

 of communication (commissures; commissura, a joining). The office 

 of these commissures is the association in function of the two sym- 

 metrical portions. Hence we arrive at the general and important con- 

 clusion, that the brain among the lower animals consists of primitive 

 cords, primitive ganglia upon those cords, and commissures which con- 

 nect the substances of the adjoining ganglia, and associate their 

 functions. 



In the development 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 become cemented upon the middle line, form- 

 ing 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 

 development, which arches backwards and conceals them ; the anterior 

 pairs, at first very small, decrease in size and become almost lost in the 

 increased development of the preceding pairs. 



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

 gressive development observed in the lower animals ; the human brain 

 is passing through the phases of improving development, which dis- 

 tinguish the lowest from the lower creatures : and we are naturally led 

 to the same conclusion with regard to the architecture of the human 

 brain, that we were led to establish as the principle of development in 



