362 NERVOUS SYSTEM DEVELOPMENT. 



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

 higher in the animal scale, and knots or ganglia are developed on one ex- 

 tremity of this cord ; such is the most rudimentary condition of the brain 

 in the lowest forms of vertebrata. In the lowest fishes the anterior ex- 

 tremity 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 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 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 \ve 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 symmetrical portions. 

 Hence we arrive at the general and important conclusion, that the brain, 

 among the lower animals, consists of primitive cords, primitive ganglia 

 upon those cords, and commissures which connect the substance of adjoin- 

 ing ganglia, and associate their functions. 



In the development of the cerebro-spinal axis in man, the earliest indi- 

 cation of the spinal cord is presented under the form of a pair of minute 

 longitudinal filaments placed si'de by side. Upon these, towards the an- 

 terior 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 corres- 

 pond w r ith the direction of the future cranium. The posterior pair soon 

 become cemented on the middle line, forming a single ganglion ; the se- 

 cond pair also unite with each other ; the third and fourth pairs, at first 

 distinct, are speedily veiled by a lateral development, which arches back- 

 wards and conceals them ; the anterior pair, at first very small, decrease 

 in side, and become almost lost in the increased development of the pre- 

 ceding pairs. 



We see here a chain of resemblances corresponding with the progressive 

 development observed in the lower animals ; the human brain is passing 

 through the phases of improving development, which distinguish 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 the inferior creatures, namely, 

 that it is composed of primitive cords, primitive ganglia upon those cords, 

 commissures to connect those ganglia, and developments from those 

 ganglia. 



In the adult, the primitive longitudinal cords have become cemented 

 together, to form the spinal cord. But, at the upper extremity, they se- 

 parate from each other under the name of crura cerebri. The first pair 

 of ganglia, developed from the primitive cords, have grown into the cere" 

 bellum ; the second pair (the optic lobes of animals) have become the 

 corpora auadrigemina of man. The third pair, the optic thalami, and the 

 fourth, the corpora striata, are the basis of the hemispheres, which, the 



