Houser, The Neurones of a Selachian. 67 
3. The Nucleus Neuroporicus : ‘ : 3 : 151 
4. The Pallium . 5 ; . : 5 ra U7. 
a. Neurones of the Tere Pallii : : : : 152 
b. Associative and Commissural Neurones . : Sree 
c. CajaL Neurones , ; : é : 154 
d. General Considerations on He Pallium : é Fe IAG 
5. Supporting Elements - : : : ; ; 156 
6. Summary of the Forebrain : : : : 157 
SECTION IX. GENERAL SUMMARY AND Coscrmson - : : Se SS! 
SECTION X. LITERATURECITED . E : ; : : ; 164 
SECTION XI. DESCRIPTION OF THE FIGURES : ; 5 - ine 169 
SEcTION I. INTRODUCTORY. 
The student of mammalian neurology has his attention 
fixed on a mechanism of surpassing complexity. In the pur- 
suit of his work, he is continually touching problems, both mor- 
phological and physiological, which frequently transcend all his 
powers. But the complex nervous skein which he seeks to 
unravel is merely the final member:of a series reaching back- 
ward through ever simpler and simpler conditions to the organ- 
ization of the primitive vertebrate. In other words, the mam- 
malian brain is the product of endless modifications wrought 
in the original plan of structure by the continual adjustment 
of nervous mechanisms to the play of a shifting environ- 
ment. What the architecture of the ancestral vertebrate 
nervous system may have been we can never hope to know 
from actual observation. Fortunately, there are simple verte- 
brates existing to-day which retain many features of primitive 
nervous organization. To such animals, the student of neurol- 
ogy must ever turn for the solution of the problems which vex 
him in higher fields. One of these simple vertebrates is repre- 
sented by a selachian of the modern seas, somewhat specialized 
in certain directions, of course, but retaining, withal, much of 
the archaic nervous organization from which higher brains have 
been gradually evolved. A study of sucha simple brain as 
that of a selachian constitutes, therefore, a necessary introduc- 
tion to the more highly differentiated nervous systems of birds 
and mammals. 
Our knowledge of the nervous system is so peculiarly de- 
