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 THIRD VENTRICLE. 373 



two rounded elevations called corpus geniculatum externum and 

 internum. The corpus geniculatum externum is the larger of the 

 two, and of a grayish colour ; it is the principal origin of the optic 

 nerve. Inferiorly, the thalami are connected with the corpora albi- 

 cantia by means of two white bands, which appear to originate in 

 the white substance uniting the thalami to the corpora striata. In 

 their interior the thalami are composed of white fibres mixed with 

 gray substance. They are essentially the inferior ganglia of the 

 cerebrum. 



The Third ventricle is the fissure between the two thalami optici. 

 It is bounded above by the under surface of the velum interpositum, 

 from which are suspended the choroid plexuses of the third ventricle. 

 Its floor is formed by the anterior termination of the corpus callosum, 

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

 rally it is bounded by the thalami optici and part of the corpora 

 striata; anteriorly by the anterior commissure and crura of the 

 fornix ; and posteriorly by the posterior commissure and the iter e 

 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 rounded white cord, which enters 

 the corpus striatum at either side ; the middle, or soft commissure 

 consists of gray matter, and is very easily broken down ; it connects 

 the adjacent sides of the thalami optici : and the posterior commis- 

 sure is a flattened white cord, connecting the two thalami optici 

 posteriorly. 



Between the anterior and middle commissure is the space called 

 foramen commune anterius, which, from leading downwards into the 

 infundibulum, is also designated iter ad infundibulum. The crura 

 of the fornix descend through this space, surrounded by gray matter, 

 to the corpora albicantia. Between the middle and posterior com- 

 missure is the foramen commune posterius, from which a canal leads 

 backwards to the fourth ventricle, the iter a tertio ad quartum ventri- 

 culum. 



Behind the third ventricle is placed the quadrifid ganglion, called 

 optic lobes in the inferior animals, and corpora quadrigemina in man. 

 The two anterior of these bodies are the larger, and are named 

 nates; the two posterior, testes. Their base is perforated from 

 before backwards by a tubular canal, which serves to communicate 

 the third and fourth ventricles, and is thence named the iter e tertio 

 ad quartum ventriculum, or aqueduct of Sylvius. Resting upon the 

 corpora quadrigemina and surrounded by a sheath of pia mater, 

 obtained from the velum interpositum, with which it is liable to be 

 torn off unless very great care be used, is the pineal gland. 



The Pineal gland consists of soft gray substance, and is of a 

 conical form ; hence one of its synonymes, conarium. It contains 

 in its interior several brownish granules, which are composed of 

 phosphate and carbonate of lime. It is connected to the thalami 



