THE FROG 51 
endolymph space of the outer covering membrane (dura mater) 
of the spinal cord, filled with fine granules of calcium carbonate. 
Beginning at the posterior end, the divisions of the brain as 
viewed from the dorsal aspect are: 
(a) The medulla oblongata. This may be easily recognized 
by the triangular cavity on its dorsal surface. This cavity is 
the fourth ventricle of the brain and is continuous with other 
cavities (ventricles) in the brain and with a central canal in the 
cord. The cavity of the fourth ventricle is roofed over by 
vascular epithelium which is thrown into numerous inturned 
folds forming the posterior choroid plexus. 
(6) The cerebellum is the transverse fold lying in front of 
the medulla. In the frog it is very small. 
(c) The optic lobes are two rounded masses immediately in 
front of the cerebellum. 
(d) The thalamencephalon is the portion between the optic 
lobes and the cerebral hemispheres (see below). Its cavity is 
known as the third ventricle; the roof of the latter is very thin 
and vascular and forms the anterior choroid plexus. Near the 
center of this arises a slender lobe, the pineal body or epiphysis, 
which extends forward over the thalamencephalon; a_ smal! 
parietal nerve runs along the dorsal surface of the epiphysis and 
passes through the skull to the brow spot. Both epiphysis and 
parietal nerve are frequently torn away from the brain when the 
roof of the cranium is removed. In front of the epiphysis and 
between the posterior ends of the cerebral hemispheres is the 
paraphysis, a vascular and probably glandular outpocketing of 
the roof of the thalamencephalon. 
(e) The cerebral hemispheres are two anterior swellings sepa- 
rated from the forward-projecting olfactory lobes by a slight 
constriction. The olfactory lobes are continued forward as the 
olfactory nerves (first pair of cranial nerves). 
Draw a dorsal view of the central nervous system, twice 
natural size. Make this drawing in the center of a clean page, so 
that other features may be added later. 
After studying the peripheral nervous system (see below), 
remove the brain and spinal cord along with the roots of all the 
nerves so far as possible. Begin at the anterior end; cut through 
