238 A MANUAL OF ANATOMY. 



probably in the amygdaloid nucleus as well. Its fibres are 

 connected with the fibres which constitute the middle root 

 of the olfactory tract. The anterior commissure is devel- 

 oped from the lamina terminalis. 



The basal gray commissure is that portion of the floor 

 of the third ventricle which is formed by the lamina cine- 

 rea, tuber cinereum, and posterior perforated space. 



The bundle of Vicq d' Azyr is a band of white fibres 

 which passes from a corpus albicans vertically into the an- 

 terior extremity (anterior tubercle) of the optic thalamus. 

 It was formerly described as a continuation of the anterior 

 pillar of the fornix, which, after reaching the base of the 

 brain, suddenly bent upward on itself (forming the corpus 

 albicans) and passed to the optic thalamus. The optic 

 thalami, pineal gland, pituitary body, and the upper arm of 

 the optic nerve arise from the walls of the interbrain, while 

 its central cavity remains as the third ventricle. 



DISSECTION. 



After studying the crura cerebri externally, transverse sections should be 

 made of them as far back as the posterior quadrigeminal bodies. 



The Crura Cerebri. Figs. 29, 38, 39, 40, 45, 46, 50, 53, 

 These are the rounded bundles of fibres which pass down- 

 ward from the under surface of the cerebrum. They are 

 oval in cross section and united along the posterior half of 

 their opposing surfaces, while the anterior portions are 

 separated, and in the interval between them is found the 

 posterior perforated space and the third nerves. The crura 

 cerebri are continuous posteriorly, around the aqueduct of 

 Sylvius, with the quadrigeminal lamina. 



Portions of the crura cerebri have been named for pur- 

 poses of distinction. On a transverse section there will be 

 seen a dark or black semilunar-shaped nucleus, which is 



