FOURTH VENTRICLE. 379 



the lateral and third ventricles, and it transmits superiorly the choroid 

 plexus and the venae corporum striatorum. The foramen commune ante- 

 rius is also termed, iter ad infundibulumj from leading downwards to the 

 funnel-shaped cavity of the infundibulum. The crura of the fornix are 

 embedded in the lateral walls of the foramen commune, and are concealed 

 from view in this situation by the layer of grey substance which lines the 

 interior of the third ventricle. If the crura be slightly separated, the an- 

 terior commissure will be seen immediately in front of them, crossing from 

 one corpus striatum to the other. The space between the middle and 

 posterior commissure is the foramen commune posterius ; it is much shal 

 lower than the preceding, and is the origin of a canal, the aqueduct of 

 Sylvius or iter a tertio ad quartum ventriculum, which leads backwards 

 beneath the posterior commissure and through the base of the corpora 

 quadrigemina to the upper part of the fourth ventricle. 



CORPORA QUADRIGEMINA. The corpora quadrigemina, or optic lobes, 

 are situated immediately behind the third ventricle and posterior commis- 

 sure ; and beneath the posterior border of the corpus caliosum. They 

 form, indeed, at this point, the inferior boundary of the transverse fissure 

 of the hemispheres, the fissure of Bichat. The anterior pair of these bodies 

 are grey in colour, and are named nates : the posterior pair are white and 

 much smaller than the anterior ; they are termed testes. From the nates 

 on each side may be traced a rounded process (brachium anterius) which 

 passes obliquely outwards into the thalamus opticus ; and from the testis 

 a similar but smaller process (brachium posterius) which has the same 

 destination. The corpus geniculatum internum lies in the interval of 

 these two processes where they enter the thalamus, and behind the bra- 

 chium posterius is a prominent band (laqueus) which marks the course of 

 the superior division of the fasciculus olivaris. The corpora quadrigemina 

 are perforated longitudinally through their base by the aqueduct of Syl- 

 vius ; they are covered in partly by the pia mater and partly by the velum 

 interpositum, and the nates form the base of support of the pineal gland. 



PINEAL GLAND. The pineal gland is a small reddish grey body of a 

 conical form (hence its synonym conarium\ situated on the anterior part 

 of the nates and invested by a duplicature of pia mater derived from the 

 under part of the velum interpositum. The pineal gland, when pressed 

 between the fingers is found to contain a gritty matter (acervulus) com- 

 posed chemically of phosphate and carbonate of lime, and is sometimes 

 hollow in the interior. It is connected to the brain by means of two me- 

 dullary cords called peduncles and a thin lamina derived from the posterior 

 commissure ; the peduncles of the pineal gland are attached to the thalami 

 optici, and may be traced along the upper and inner margin of those 

 bodies to the crura of the fornix with which they become blended. From 

 the close connexion subsisting between the pia mater and the pineal gland, 

 and the softness of texture of the latter, the gland is liable to be torn away 

 in the removal of the pia mater. 



Behind the corpora quadrigemina is the cerebellum, and beneath the 

 cerebellum the fourth ventricle. The student must therefore divide the 

 cerebellum down to the fourth ventricle, and turn its lobes aside to ex- 

 amine that cavity. 



FOURTH VENTRICLE. The fourth ventricle (sinus rhomboidalis) is the 

 ventricle of the medulla oblongata, upon the posterior surface of which, 

 and of the pons Varolii, it is placed. Jt is a lozenge-shaped cavity, 



