528 NERVOUS SYSTEM. 



crura of the fornix, and part of the anterior commissure ; behind, by the posterior 

 commissure, and the iter e tertio ad quartum ventriculum. 



The cavity of the third ventricle is crossed by three commissures, named, from 

 their position, anterior, middle, and posterior. 



The anterior commissure is a rounded cord of white fibres, placed in front of the 

 anterior crura of the fornix. It perforates the corpus striatum on either side, and 

 spreads out into the substance of the hemispheres, over the roof of the descending 

 horn of each lateral ventricle. 



The middle or soft commissure consists almost entirely of gray matter. It 

 connects together the thalami optici, and is continuous with the gray matter lining 

 the anterior part of the third ventricle. 



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

 fibres, connecting together the two thalami 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 two oval 

 apertures, one on either side, the foramina of Monro, through which the third 

 communicates with the lateral ventricles. Behind, is a third opening leading into 

 the fourth ventricle by a canal, the aqueduct of Sylvius or iter e 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 infundi- 

 bulum, the iter ad infundibulum. 



The lining membrane of the lateral ventricles is continued through the foramina 

 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 thalami, extending 

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

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



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

 quadrigernina ; and resting upon these, the pineal gland. 



The pineal gland, so named from its peculiar shape (pinus, the fruit of the fir), 

 called also the conarium, 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 mater, 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 acermdus cerebri, composed of phosphate 

 and carbonate of lime, 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- 

 {jcmina are exposed. 



