598 NERVOUS SYSTEM. 



The posterior commissure, smaller than the anterior, is a flattened white band 

 of fibres, connecting together the two thalarni optici posteriorly. It bounds 

 the third ventricle posteriorly, and is placed in front of and beneath the pineal 

 gland, above the opening leading to the fourth ventricle. 



The third ventricle has four openings connected with it. In front are the 

 two oval apertures of the foramen of Monro, one on either side, through which 

 the third communicates with the lateral ventricles. Behind is a third opening 

 leading into the fourth ventricle by a canal, the aquasduct of Sylvius, or iter a 

 tertio ad quartum ventriculum. The fourth, situated in the anterior part of the 

 floor of the ventricle, is a deep pit, which leads downwards to the funnel- 

 shaped cavity of the infundibulum (iter ad infundibulum). 



The lining membrane of the lateral ventricles is continued through the 

 foramen of Monro into the third ventricle, and extends along the iter a tertio 

 into the fourth ventricle; at the bottom of the iter ad infundibulum it ends in 

 a cul-de-sac. 



Gray matter of the third ventricle. A layer of gray matter covers the greater 

 part of the surface of the third ventricle. In the floor of this cavity it exists in 

 great abundance, and is prolonged upwards on the sides of the thalarni, extend- 

 ing across the cavity as the soft commissure; below, it enters into the corpora 

 albicantia, and surrounds in part the anterior pillars of the fornix. 



Behind the third ventricle, and in front of the cerebellum, are the corpora 

 quadrigemina; and resting upon these, the pineal gland. 



The pineal gland (conarium), so named from its peculiar shape (pinus, a fir- 

 cone), is a small reddish-gray body, conical in form, placed immediately behind 

 the posterior commissure, and between the nates, upon which it rests. It is 

 retained in its position by a duplicature of pia matter, derived from the under 

 surface of the velum interpositum, which almost completely invests it. The 

 pineal gland is about four lines in length, and from two to three in width at its 

 base, and is said to be larger in the child than in the adult, and in the female 

 than in the male. Its base is connected with the cerebrum by some transverse 

 commissural fibres, derived from the posterior commissure; and by four slender 

 peduncles, formed of medullary fibres. Of these the two superior pass forwards 

 upon the upper and inner margin of the optic thalami to the anterior crura of 

 the fornix, with which they become blended. The inferior peduncles pass 

 vertically downwards from the base of the pineal gland, along the back part of 

 the inner surface of the thalami, and are only seen on a vertical section through 

 the gland. The pineal gland is very vascular, and consists chiefly of gray 

 matter, with a few medullary fibres. In its base is a small cavity, said by some 

 to communicate with that of the third ventricle. It contains a transparent 

 viscid fluid, and occasionally a quantity of sabulous matter, named accrvulus 

 cerebri, composed of phosphate and carbonate of lirne, phosphate of magnesia 

 and ammonia, with a little animal matter. These concretions are almost 

 constant in their existence, and are found at all periods of life. When this body 

 is solid, the sabulous matter is found upon its surface, and occasionally upon 

 its peduncles. 



On the removal of the pineal gland and adjacent portion of pia mater, the corpora quadri- 

 gemina are exposed. 



The corpora or tuberoula quadrigemina (optic lobes} are four rounded eminences 

 placed in pairs, two in front, and two behind, and separated from one another 

 by a crucial depression. They are situated immediately behind the third ven- 

 tricle and posterior commissure, beneath the posterior border of the corpus 

 callosum, and above the iter a tertio ad quartum ventriculum. The anterior pair, 

 the nates, are the larger, oblong from before backwards, and of a gray color. 

 The posterior pair, the testes, are hemispherical in form, and lighter in color 

 than the preceding. They are connected on each side with the tbalamus opti- 

 cus, and commencement of the optic tracts, by means of two white prominent 

 bands, termed brachia. Those connecting the nates with the thalamus (brachia 



