THE CENTRAL NERVOUS SYSTEM. 357 



(foramen of Monroe). Its cranial boundary forms in the middle 

 line a thin plate, the lamina terminalis (W), 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, ^'), 

 the pineal body (j), the choroid plexus of the third ventricle 

 (w), the columns of the fornix (f), 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, /). 



5. Telcncep/ialon. 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 



