12 ANATOMY OF THE CENTRAL NEETOUS SYSTEM. 



through the studies of S. E. y Cajal, Van Gehuchten, and Brandis, some- 

 thing at least is known of the mesencephalon and of the origin of nerve- 

 tracts. The most important study of the avian brain is that of Bumm. 



The establishing of homologies has been more and more facilitated 

 through Burkhardt's studies in comparative anatomy and through the em- 

 bryological work of Kupffer and of His. Through these studies we first 

 learned to recognize the importance in comparative studies of invaginations 

 and evaginations observable upon the brain. 



Comparative anatomy has not, wp to the present time, increased our 

 knowledge of the course of nerve-tracts as much as we were justified in 

 expecting. The interest in the minute structure has been slight compared 

 with that in the determination and description of the outer form, notwith- 

 standing the fact that the former is the kernel, while the latter is but the 

 outer shell. This may be attributed to the inadequacy of the methods 

 which were at command. Only a few were able clearly to recognize, in the 

 labyrinth of tracts, — present even in the lowest invertebrates, — single fas- 

 ciculi, or to differentiate single ganglia and nerve-origins. Simple and clear 

 as are the outer features of the lower vertebrate brain, the inner structure 

 is, nevertheless, hardly less complicated, especially in those regions posterior 

 to the Thalamencephalon, than in the mammals themselves. The cells and 

 nerve-tracts which are involved in the simplest motor, sensory, or psychical 

 apparatus must, indeed, be everywhere the same, and they are not alto- 

 gether simple and clear even in so low a form as the larva of the cyclostomii. 

 That the solution of the problem might be more nearly reached, the 

 author has endeavored to combine the comparative anatomical methods with 

 the study of nerve-sheath development. 



We are now able to stain and to trace every individual nerve-sheath. In 

 fact, the comparative embryological method succeeded finally in finding in 

 the embryos of the lower vertebrates the very simple relations sought for 

 and to fix definitely upon a number of nerve-tracts as common to all verte- 

 brates. 



Of the very greatest importance for our general idea of the nervous 

 system were the discoveries which followed upon the Golgi impregnation 

 method and upon the Ehrlich method of vital staining with methyl-blue. 



Through this amplification of the method we are finally in a position 

 to reach a clearer understanding of the relations of the cells to each other 

 and to obtain a more definite idea of the minute structure in general. 



To these methods we are indebted for the most important discoveries 

 made in recent years, for the insight into the previously-unknowji nervous 

 system of the invertebrates, and for Eetzius' happy discovery: that the en- 

 tire nervous system of an invertebrate may, under certain circumstances, le 

 seen in its complete connections. Through the excellent work of this in- 



