24 



thalamus 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 olfac- 

 tory 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 prosence- 

 phalon 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 pas- 

 sage through the mesencephalon, the aqueduct of Sylvius (iter, 

 mesocoel) . 



The optocoels are large cavities within the optic lobes which 

 open into the aqueduct of Sylvius. 



A large metacoel in the metencephalon opens into the myelo- 

 coel. The myelocoel is also continued into the corpora restiforma; 

 posteriorly it joins the central canal which extends down the center 

 of the spinal cord. 



DISSECTION OF THE EYE. Remove one of the eyes from its orbit, 

 and divide it into inner and outer halves by an equatorial cut around 

 the eyeball (not directly through it, as this tears the lens from its 

 fastenings). Place the halves under water and observe: 



In the inner half: 



The posterior chamber, the cavity of the eyeball which has been 

 opened. During life it is filled by a gelatinous substance, the 

 vitreous humor. 



The retina, a delicate yellowish-white membrane lining the 

 interior of the eye, loosely attached to the outer coats except at the 

 point of entrance of the optic nerve. 



The choroid coat, a thin, black membrane outside the retina. It 

 can be pulled away from the outer coat quite easily except near the 

 optic nerve. 



The sclerotic coat, the outer coat of the eye. This is composed 

 of connective tissue having an almost cartilaginous consistency, is 

 only slightly pigmented, and is somewhat translucent. The muscles 

 of the eye are inserted upon the sclerotic. 



In the outer half: 



The ora serrata, an irregular line along which the retina ends. 



The iris, a fold of the choroid extending inward like a shelf, and 



