The =third ventricle= (Fig. 143, _h_; Fig. 141, _v_) is a very narrow slit-like space, of considerable extent dorsoventrally, but less than a millimeter in width; it lies between the medial ends of the thalami (Fig. 141), and extends ventrad into the tuber cinereum, and thence into the infundibulum (Fig. 143, _g′_). Dorsad it extends into the corpus pineale (Fig. 143, _y_). The third ventricle communicates caudally near its dorsal border with the aqueductus cerebri (_j_); craniolaterally with the lateral ventricles, through the =interventricular foramen= (foramen of Monroe). Its cranial boundary forms in the middle line a thin plate, the =lamina terminalis= (_d_), which is, morphologically, 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 (_f_). The boundaries of the third ventricle, in order, are as follows, beginning dorsocraniad of the communication with the aqueductus cerebri: the posterior commissure (Fig. 143, _z′_), 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, _t_). 5. _Telencephalon._--The telencephalon includes the two cerebral hemispheres. The name =cerebrum= is also applied to this portion of the brain; frequently, however, the name cerebrum 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, _p_; Fig. 147; Figs. 149-152, _a_), a broad transverse band of white fibres connecting the two hemispheres dorsad of the ’tween-brain. Ventrad of the corpus callosum another secondary union has resulted in the production of the =fornix= (Fig. 143, _u_).