152 
IOWA ACADEMY OF SCIENCES. 
reasons modern neurological investigation has been largely con- 
cerned with the mammalian brain in general and the human 
brain in particular. But the structures here are highly special- 
ized, and their significance cannot always be thoroughly under- 
stood. In order to unravel the tangled threads of the complex 
neurological skein, the study of some primitive type of brain 
is an absolute necessity. The brain of the shark is the one to 
which we naturally turn for this purpose because of the mor- 
phological position which it occupies. 
The several parts of the brain are arranged in almost perfect 
longitudinal series, and are well separated from each other. 
The prosencephalon is a relatively large, unpaired, globular mass. 
Its ventricle is imperfectly divided into lateral ventricles. A 
very prominent olfactory apparatus projects anteriorly. On the 
dorsal surface there are to be seen two slight swellings which 
may be taken as the anlages of the cerebral hemispheres of 
higher forms. The thalamencephalon is narrow, open dorsally, 
and the choroid plexus passes in to form a thin roof. The 
epiphysis arises just behind this point. It is long and slender, 
and ends in a dilation which is attached to the membranous roof 
of the skull. Both the optic lobes and the cerebellum retain the 
primitive condition of hollow outgrowths. The cerebellum is 
relatively quite large, and is thrown into transverse folds. The 
large size is evidently related to the swimming habits of the 
animal. The fourth ventricle of the medulla oblongata is widely 
open. Its sides are thickened, and project anteriorly as the 
restiform bodies. 
The microscopic structure of the shark’s brain was investi- 
gated by a few of the older workers, Viault, Rohon, and San- 
ders requiring especial mention here. The application of silver 
impregnation by Golgi to the study of nerve cells has, however, 
opened a new era in neurology, and has made necessary the 
reinvestigation of every species of nervous system. While the 
older methods of research had brought out certain general 
facts about the structure of the shark’s brain, it is only through 
the application of the Golgi method that we can hope to acquire 
a thorough knowledge as to its ultimate cellular structure. I 
will enumerate briefly the most important results which I have 
already reached. 
In the fore-brain the nerve ceils are large and very conspicu- 
ous. They are not arranged in layers, neither do they have a 
pyramidal form. The prevailing type presents an oblong cell 
