278 ANIMAL PHYSIOLOGY. [LESSON 86. 



head, and abundant organs of special sense, a well developed supra- 

 oesophageal ganglion is found. 



1249. From these animals we make a rapid ascent to the fishes ; 

 in the lowest of these, the anterior portion of the double chord dis- 

 plays a succession of five pairs of ganglia. 



1250. The higher fishes, and the amphibious reptiles, 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 un- 

 folded 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. 



1251. This condition of the brain carries us upwards in the scale 

 of being, even to the mammalia ; in the dog, or cat, for example, 

 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 primitive ganglia, concealed by an additional development. 



1252. 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, or commissures. The 

 oflice of these commissures is the association in function of the two 

 symmetrical portions. 



1253. Carpenter, in his human physiology, has truly said, " Hence 

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

 among the lower animals consists of primitive chords, primitive 

 ganglia upon those chords, and commissures which connect the 

 substances of the adjoining ganglia, and associate their actions." 



1254. In the development of the cerebro-spinal axis of man, the 

 earliest indication of the spinal chord is presented under the form of 

 a pair of minute longitudinal filaments placed side by side. 



1255. 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. 



1256. The posterior pair soon becomes cemented upon the mid- 

 dle 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 development, which arches backwards and con- 

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

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

 ceding pairs. 



