1>EVELOPEMENT OF THE tfERVOUS SYSTEM. 131 



structure. In Zoophytes of a little higher organization, 

 the nervous matter appears in the form of minute and 

 delicate threads arranged around the principal organs of 

 nutrition, with other threads proceeding from them in a 

 radiated mannef, to go to the'different parts of the body. 

 In the next orders we find a series of little knots of ner- 

 vous matter, called ganglia, connected by two nervous 

 threads, still placed around the organs of nutrition. In 

 the earth worm belonging to a still higher order, the ner- 

 vous thread is single, instead of double as before, and 

 the ganglia instead of appearing independent bodies, are 

 merely swellings of this thread, from which proceed sin- 

 gle and double pairs of threads alternately. In the first 

 orders which possess senses, we find the anterior ganglia 

 enlarged and sending out filaments to supply the organs 

 of sense. Thus, as the organs of the animals increase, 

 the nervous threads, or nerres we may as well call them, 

 increase in number and perfection, the principal mass 

 still being placed around the oesophagus. In the fishes 

 we meet for the first time, the most remarkable modifi- 

 cation which the system undergoes. The main nervous 

 cord becomes enclosed in a long case or canal, and take 

 the name of spinal marrow, from the long canal which is 

 called the spine. But this is not all : at the summit of 

 the spinal marrow, are observed several large ganglia 

 connected together but quite distinct from one another. 

 This constitutes the brain, and here you may see 

 it in this cut. In the brain of the reptiles, we find 

 an addition of two more ganglia. In the birds, 

 these last ganglia are much developed, and in the 

 mammiferous tribes, the brain, instead of being 

 composed of separate ganglia, has the appearance 

 of a single organ, whose parts are no longer divi- 

 sible into distinct bodies. 

 Emily. This cut, then, according to your theory, 

 will also represent the brain and spinal marrow of man, 

 at one period of his existence, will it not ? 



