74 Journal of Comparative Neurology. 



sentials of arrangement from those with which we are famiHar in 

 the mampials and man. No one would assume that the cortex- 

 less fish is lame or without sensation or blind, yet it lacks those 

 tracts which in higher animals conduct from the first termini of 

 the motor, sensory and cranial nerves to the cortex. The sole 

 conclusion which is warranted is that the first stations of these 

 nerves suffice to exert those functions which we recognize as 

 characteristic of the central organs. 



As one ascends the scale there is added (I use the word 

 " added " purposely) to the structures already existing a cortex. 

 We are able to clearly distinguish the cortex from all other parts 

 of the brain (by the form and arrangements of the cells) since 

 we possess the Golgi method. Such cells appear first in an irreg- 

 ular and sporadic manner in the amphibia. In the reptilia we 

 find a well-defined, regularly arranged cortex clothing the mantle. 

 From it arise a few fibres which connect the mantle with the 

 deeper centres. Even among the mammals the cortex is, as 

 well known, exceedingly variable in its extent. Its amount in 

 rodents and many other mammals seems quite small when com- 

 pared with that of the thalamus and mid-brain ganglia. It is 

 not until the higher mammals are reached that the cortical layer 

 of the cerebrum gradually acquires its extensive development 

 which, through the magnitude of the connections which it con- 

 tains and especially the radiations which pass to the thalamus, 

 produces the familiar convolutions. The cortex is, therefore, 

 not a necessary element in the concept of a vertebrate brain, it 

 is of gradual development and continues to increase in size in 

 the grqup of mammals itself. 



The latest to develop is that part of the cortex which, with 

 its radiations, is called the frontal lobe. In the carnivora it is 

 still very small, and even in the ape it has relatively slight devel- 

 opment, while in the anthropoids, those apes which most nearly 

 approach man, there are remarkable differences between the 

 frontal lobe and that of the lowest races of men. The rate of 

 development is not the same for all parts of the cortex. Those 

 parts of the cortex pertaining to the olfactory sense, the mar- 

 ginal gyre and ammonshorn, may attain an enormous develop- 



