THE CENTRAL NERVOUS SYSTEM. 357 
(foramen of Monroe). Its cranial boundary forms in the middle 
line a thin plate, the lamina terminalis (7), which is, morpho- 
logically, the cranial termination of the cerebrospinal axis; it 
lies at the bottom of the deep fissure between the cerebral 
hemispheres. At the dorsal border of the lamina terminalis is 
a strong transverse band of fibres connecting the two sides of 
the brain; this is the anterior commissure (c). The cavity of 
the third ventricle is much encroached upon by the meeting 
and secondary union of the two thalami across the middle line, 
forming the massa intermedia (/). 
The boundaries of the third ventricle, in order, are as fol- 
lows, beginning dorsocraniad of the communication with the 
aqueductus cerebri: the posterior commissure (Fig. 143, 2’), 
the pineal body (y), the choroid plexus of the third ventricle 
(w), the columns of the fornix (v), the anterior commissure 
(c), the lamina terminalis (d@), the tuber cinereum with the 
infundibulum (g’), the substantia perforata posterior, and the 
midbrain. The lateral boundaries are formed by the thalami 
(Fig. 141, 2). 
5. Yelencephalon.—The telencephalon includes the two 
cerebral hemispheres. The name cerebrum is also applied to 
this portion of the brain; frequently, however, the name cere- 
brum is used as signifying the entire mass of the brain craniad 
of the rhombencephalon,—therefore including mesencephalon, 
diencephalon, and telencephalon. The term will here be used 
as synonymous with telencephalon. 
The cerebral hemispheres arise as two lateral outgrowths 
from the cranial end of the primitive forebrain. They have 
undergone great increase in size in the course of evolution, so 
as to form the larger part of the brain; at the same time 
important modifications of structure have taken place. In 
the original condition the medial faces of the two hemispheres 
are not connected, the two being separate outgrowths of the 
*tween-brain, and connected only through the latter. But 
secondary connections have been formed across the fissure 
between the two hemispheres, resulting in the production of 
the corpus callosum (Fig. 143, £; Fig. 147; Figs. 149-152, a), 
a broad transverse band of white fibres connecting the two 
