Identify and examine the ventral parts of the brain. Note the considerable lateral compression of the mesencephalon. The optic nerves cross beneath the diencephalon, forming the _optic chiasma_. From the sides of the chiasma slightly elevated _optic tracts_, formed by the fibres of the optic nerves, can be traced into the optic lobes. Back of the optic chiasma the projecting ventral portion of the diencephalon forms the _hypothalamus_. The posterior lobe of this structure is the _hypophysis_ or _pituitary body_. The oculo-motor nerves emerge over the posterior end of the hypothalamus. The ventral portion of the mesencephalon is formed by the _cerebral peduncles_ (_crura cerebri_), columns of fibres passing between the myelencephalon and telencephalon. The abducens nerves arise on the ventral surface of the myelencephalon near the midline and just back of a line connecting the roots of the auditory nerves. The _internal carotid arteries_ reach the brain at the sides of the hypothalamus. Branches are sent upward and forward over the surface of the brain. Anastomoses between the vessels of the opposite sides are formed anterior to the optic chiasma. The main branches of the carotids pass backward along the sides of the hypothalamus and unite behind this organ. The median artery thus formed runs along the ventral surface of the myelencephalon and the spinal cord. Numerous transverse vessels are given off to the myelencephalon. Identify the roots of the remaining cranial nerves. CAVITIES OF THE BRAIN. Divide the brain into exactly equal halves by a vertical longitudinal cut. Each lobe of the prosencephalon contains a large cavity. These are the _prosocoels_. They are commonly known either as the lateral ventricles, or the left cavity as the first ventricle and the right as the second ventricle. The prosocoels are continued into the olfactory lobes, these portions being known as rhinocoels. The _thalamocoel_ is the cavity within the diencephalon, often called the third ventricle. The prosocoels communicate with the thalamocoel by lateral openings, the _foramina of Monro_. The roof of the thalamocoel is very thin and is non-nervous; it is frequently torn during the early dissection. Where the lobes of the prosencephalon meet the dorsal wall of the diencephalon this thin roof is pushed into the prosocoels, carrying with it the pia mater and its blood vessels, and thus forms vascular ingrowths known as the _choroid plexi_. The thalamocoel continues above into the epiphysis and below into the hypothalamus. The _myelocoel_ is the large cavity of the myelencephalon. It also is frequently apparently open to the exterior at the posterior end by the accidental breaking of the thin, non-nervous dorsal wall of this region. The myelocoel is also known as the fourth ventricle. The thalamocoel and myelocoel are connected by a narrow passage through the mesencephalon, the _aqueduct of Sylvius_ (_iter, mesocoel_).