452 THE BRAIN 



BASE OF BRAIN. 



Fossa Interpeduncularis. When the membranes are re- 

 moved from the base of the brain, \\\e pedunculi cerebri (O.T. 

 crura\ two large rope-like strands, will be seen issuing from the 

 upper aspect of the pons. Placed close together as they 

 emerge from the pons, they diverge as they proceed upwards 

 and anteriorly, and, finally, each peduncle disappears into the 

 corresponding half of the cerebrum. Turning round the 

 lateral aspect of each peduncle, where it passes into the 

 cerebrum, will be seen a flattened band, called the optic tract. 

 These bands converge as they pass anteriorly, and are finally 

 joined together by a short transverse commissural portion, 

 termed the optic chiasma. This chiasma is placed below the 

 posterior end of that portion of the longitudinal fissure which 

 intervenes between the inferior surfaces of the frontal lobes 

 of the cerebrum. The optic nerves run antero-laterally from 

 the chiasma. 



The cerebral peduncles, the optic tracts, and the optic 

 chiasma enclose a deep rhomboidal or lozenge-shaped interval 

 on the base of the brain, which is termed the inter peduncular 

 fossa. Within the limits of this area the following parts may 

 be seen. The substantia perforata posterior bounded an- 

 teriorly by the corpora mamillaria ; anterior to the corpora 

 mamillaria is the tuber cinereum, and attached to it is the 

 infundibulum of the hypophysis. These structures take part in 

 the formation of the floor of the third ventricle of the brain. 



The oculo-motor nerves issue from the brain within the 

 interpeduncular fossa. Each nerve emerges from the medial 

 side of the corresponding pedunculus cerebri. 



Substantia Perforata Posterior (O.T. Posterior perforated 

 space). At its posterior angle, immediately anterior to the 

 pons, the interpeduncular fossa is very deep, and is roofed 

 by a layer of grey matter in which are numerous small 

 apertures. This is the substantia perforata posterior. From 

 the apertures which are dotted over its surface the postero- 

 median basal branches of the posterior cerebral artery have 

 been withdrawn. 



Corpora Mamillaria. These are two small, white, pea- 

 shaped eminences placed, side by side, anterior to the sub- 

 stantia perforata posterior. At a later stage of the dissection 



