372 ELEMENTARY ANATOMY. [LESS. 



little behind this aperture is the cut edge of a bundle of 

 transverse fibres which form what is called the soft (or middle) 

 commissure. The third ventricle is bounded above by a 

 delicate membrane, the velum interpositum, which thickens 

 behind and forms a small prominence which projects back- 

 wards and is called the pineal gland reminding us of the 

 pituitary body below. The third ventricle is bounded inferiorly 

 by the corpora mammillaria and crura cerebri (the cut sur- 

 faces of which are visible in Fig. 327 just above the pons 

 Varolii), and by the infundibulum, into which it extends. 



The cavity just described, the third ventricle, is not shut in 

 at its hinder end below, but continues on as a very narrow 

 passage (the iter a tertio ad quartum ventriculum), bounded 

 in front by the crura cerebri and behind by a layer of nervous 

 matter continuous with the pineal gland, and exhibiting the 

 cut surface of a small transverse cord (the posterior commis- 

 sure), and also two prominences in section part of the 

 corpora quadrigemina. A little lower down, this passage 

 expands into a second cavity (the fourth ventricle), bounded 

 in front by the medulla oblongata and behind by the cere- 

 bellum above, and below the cerebellum by an exceedingly 

 delicate layer of nervous tissue. 



The cerebellum in section shows singular radiating tree- 

 like ramifications of nervous substance (grey and white), due 

 to infoldings of the surface of the organ, and called the arbor 

 vita. 



Thus the extension backwards of the corpus callosum 

 and cerebrum altogether overlaps a certain portion of the 

 brain, namely, the pineal gland and parts adjacent. When 

 these are exposed by a special section the corpora quadri- 

 gemina are seen to consist of two pairs of small prominences 

 (but little different in size) placed side by side immediately 

 behind the pineal gland. The anterior pair are called the 

 nates, the posterior pair the testes. They are solid structures. 



7. OTHER SECTIONS (Figs. 328 and 329) are necessary to 

 make clear other matters. Thus, the foramen of Monro is 

 the entrance to a cavity which is placed in the cerebral hemi- 

 sphere of the same side, these two cavities constituting the 

 first and second (or two lateral} ventricles. 



The so-called foramen of Monro is, in fact, a Y-shaped 

 passage. It is single below, where it communicates with the 

 third ventricle, but divides above into two branches, one to 

 each lateral ventricle. 



Each lateral ventricle is tri-radiate and said to have three 



