282 Journal of CoMi'AJiATi\E Neurology. 



existed long before the first bii'd was evolved. If this be 

 true, then this tendency towards a progressive compactness 

 of the brain, combined with a tendency to develop all parts 

 pertaining to vision and to atrophy all parts pertaining to 

 smell, will account for all the major differences between the 

 avian and reptilian brain. 



Furthermore, within this class of animals, this progressive 

 compactness of the bi-ain is a factor of taxonomic importance. 

 So far, at least, as the major groups are concerned, a classifi- 

 cation based upon this alone is in harmony with those classi- 

 fications that are based upon a study of the structural ele- 

 ments of birds. 



2. Neurologically considered, birds are preeminently see- 

 ing animals, and all parts that appertain to vision are devel- 

 oped to a great degree. The optic nerve is the largest of all 

 the cranial nerves, and the optic lobes are complexly difl^er- 

 entiated bodies. Even the third, fourth and sixth nerves, 

 although quite small, are relatively larger than the corre- 

 sponding nerves of the mammalian brain. 



An extraordinary development of one set of organs is 

 never accomplished but at the expense of some other set. In 

 this case the organs of smell have been the martyrs. Al- 

 though in the lower avian types the olfactory lobes are paired 

 and conspicuous, yet in the highest type of birds the rhinen- 

 cephalon is a small unpaired body, which is partly embedded 

 in the base of the cerebrum. 



3. Histologically, the avian brain is composed of nerve 

 fibres, nerve cells and neuroglia. In this connection we are 

 concerned with nerve cells only. Although these cells pre- 

 sent a great diversity of forms, yet they may all be grouped 

 in the following classes: ganglionic cells, Deiter's corpuscles, 

 fusiform or flask cells, pyramidal cells, and multipolar cells. 

 The ganglionic cells are large bipolar cells, which are never 

 found outside of the root ganglia. Each extremity of each 

 of these cells is prolonged into a nerve fibre. In addition to 

 the ordinary cell wall, each cell is surrounded by a special, 



