418 CORPORA QUADRIGEMINA. 



the tuber cinereum, corpora albicantia, and locus perforatus. Late- 

 rally it is bounded by the thalami optici ; anteriorly by the anterior 

 commissure and crura of the fornix ; and posteriorly by the posterior 

 commissure and the iter a tertio ad quartum ventriculum. The third 

 ventricle is crossed by three commissures, the anterior, middle, and 

 posterior ; and between these are two spaces, called foramen commune 

 anterius and foramen commune posterius. 



The Anterior commissure is a small rounded white cord, which enters 

 the corpus striatum at either side, and spreads out in the substance of 

 the hemispheres ; the middle, or soft commissure consists of grey matter, 

 which is continuous with the grey lining of the ventricle, it connects 

 the adjacent sides of the thalami optici ; the posterior commissure, 

 smaller than the anterior, is a rounded white cord, connecting the two 

 thalami optici posteriorly. 



The space between the anterior and middle commissure is called the 

 foramen commune anterius, and is that to which Monro has given his 

 name (foramen of Monro). It is the medium of communication be- 

 tween the lateral and third ventricles and it transmits superiorly the 

 choroid plexus and the venae corporum striatorum. The foramen com- 

 mune anterius is also termed, iter ad infundibulum, from leading down- 

 wards 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 sub- 

 stance which lines the interior of the third ventricle. If the crura be 

 slightly separated, the anterior commissure will be seen crossing from 

 one corpus striatum to the other, immediately in front of them. The 

 space between the middle and posterior commissure is the foramen 

 commune posterius; it is much shallower 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 poste- 

 rior commissure ; and beneath the posterior border of the corpus callo- 

 sum. 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 pos- 

 terior pair are white and much smaller than the anterior; they are 

 termed testes. From the nates may be traced a rounded process 

 (brachium anterius) which passes obliquely outwards into the thala- 

 mus opticus ; and from the testes a similar but smaller process (bra- 

 chium posterius) which has the same destination. The corpus genicu- 

 latum internum lies in the interval of these two processes where they 

 enter the thalamus, and behind the brachium posterius is a prominent 

 band (laqueus) which marks the course of the superior division of the 

 fasciculus olivaris. The corpora quadrigemina are perforated longitu- 

 dinally through their base by the aqueduct of Sylvius; they are 



