DEVELOPMENT OF THE BRAIN AND GANGLIA. 55 



The glimpse which has been presented of the developing brain has 

 possibly already called forth the question: How does the brain grow? And 

 the comparison with the richly convoluted brain of the adult may have 

 suggested also the question: At about what time has the brain reached the 

 form and size which it retains through the major part of life or until that 

 time when advancing age induces senile decadence? 



If the brain is the organ upon the faultless performance of whose 

 function the normal psychical processes depend, it is worth our while to 

 know how long new tissue-elements may be formed and upon the in- 

 crease of what part the growth of the whole depends. Investigations which 

 might answer these important questions have made little advance. There 

 is a complete lack of investigations on the multiplication of ganglion- 

 cells in different parts of the brain in the post-embryonic period. Up 

 to the present time karyokinesis of nuclei of central cells has been very 

 infrequently found; so that it might appear as if the brain, which has 

 approximately reached the form and weight of the adult organ at about 

 the seventh year, had, by that time, established all of its ganglion-cells. 

 Schiller's proof, that the slender oculomotorius of the newborn cat contains 

 scarcely fewer fibers than the thick nerve of the adult, favors the theory of 

 an early termination of cell-formation. There are, however, opposing state- 

 ments. Thus, Kayser found in the cervical enlargement of the spinal cord 

 of the newborn only about half as many ganglion-cells as in the same 

 region of a 15-year-old boy, while the number found in the latter case was 

 little different from that found in an adult man. Also enumerations which 

 Birge and others have made on the spinal cord of the frog determined 

 positively that in these animals the ganglion-cells continue to increase for a 

 long time in adult life. 



Probably the principal post-embryonal increase of nerve-tissue in man 

 occurs in the growth of nerve-fibers and collaterals from ganglion-cells 

 already present, and ' especially of the formation of medullary substance 

 which continues throughout life. Donaldson, to whom we are indebted for 

 an excellent work on "The Growth of the Brain," came to the same 

 conclusion. 



The human cerebrum having been an important object of your previous 

 study, it has seemed profitable to describe its development somewhat in 

 detail. But since we are not here concerned with the human brain alone, 

 let us study in the brain of another vertebrate those first developmental 

 processes which are common to all vertebrates. For this the brain of a 

 reptile — the lizard — has been chosen. In this (see Fig. 35) one may readily 

 recognize the typical parts of the vertebrate brain, because even in adult 

 reptiles the relations are much simpler than in mammals. 



The middle of the figure is occupied by the cavity of Thalamen- 



