There are thus altogether five principal subdivisions of the brain, each enclosing a portion of the central cavity. These are the =myelencephalon=, the =metencephalon= (these two enclosing the =fourth= ventricle), the =mesencephalon= (enclosing the =cerebral aqueduct=), the =diencephalon= (enclosing the =third= ventricle), and the two cerebral hemispheres, constituting together the =telencephalon=, and enclosing the two =lateral ventricles=. A plan of the brain, considered as a tube enclosing cavities, is given in Figs. 139 and 140. Fig. 139 shows the plan considered as viewed from the dorsal side, Fig. 140 in a lateral view. These figures are of course pure diagrams, showing only what may be called the _plan_ of structure, and omitting all details. In the actual conditions many important modifications even of the main features of the plan are met with. In the following description the parts of the brain will be taken up in this order: A. RHOMBENCEPHALON (primitive hindbrain). 1. _Myelencephalon._ 2. _Metencephalon._ B. MESENCEPHALON (primitive midbrain). 3. _Mesencephalon._ C. PROSENCEPHALON (primitive forebrain). 4. _Diencephalon._ 5. _Telencephalon._ A. RHOMBENCEPHALON. 1. _Myelencephalon._--The myelencephalon or medulla oblongata is transitional between the spinal cord and the brain. It has in general the structural peculiarities of the cord, but these undergo in it a gradual transformation into the arrangements characteristic of the brain. It has the form of a flattened and truncated cone, which widens craniad and is limited at the cranial end by the pons (Fig. 138, _i_) ventrally and laterally; by the cerebellum (Fig. 137, _III_) dorsally. The cranial portion of its dorsal surface is overhung by the cerebellum. The points of origin of the roots of the first pair of cervical nerves (Fig. 138, _s_) may be taken as indicating the boundary between the spinal cord and the medulla oblongata; there is no other external marking to show the limits of the two. The central canal of that part of the medulla which is overhung by the cerebellum is greatly widened (Fig. 141, _h_). The cavity thus formed is flattened and triangular, with the apex of the triangle caudad, and is the caudal part of the =fourth ventricle= (Fig. 141, _h_). The roof is very thin (Fig. 143, _n_) and is intimately connected with the pia mater, so that in removing the latter the thin roof of the fourth ventricle is removed with it and the cavity of the fourth ventricle is left exposed. The thin roof of the fourth ventricle is known as the =velum medullare posterius= (Fig. 143, _n_). The ventral (anterior) fissure (Fig. 138, _p_) of the cord passes onto the medulla oblongata and ends at the pons (_i_), its end being slightly deeper and forming what is known as the =foramen cæcum= (_q_). The dorsal (posterior) sulcus (Fig. 141, _a_) is well marked; it ends at the caudal boundary of the fourth ventricle (_h_).