DISSECTION OF THE BRAIN, OR ENCEPHALON. 243 



to the crura, also recede some fibres. Tlie grey matter of the crus 

 contains nerve cells with dark pigment, and is therefore termed the 

 locus niger. ^ The crura cerebri show the superficial origin of the 3rd 

 pair of nerves. 



The Optic Tracts. These are two white cords of nerve fibres which 

 turn round the crura cerebri, and pass' forwards and inwards to meet in 

 the middle line and form by their fusion the optic commissure or chiasma. 

 This commissure rests on the sphenoid bone, in front of the pituitary 

 fossa ; and in front it gives off the diverging optic or 2nd nerves. The 

 optic tracts form the anterior boundary of the interpeduncular space. 



The Pons Tarini is the grey matter in the posterior angle of the 

 interpeduncular space. It is also known as the locus jierf or atus posticus, 

 from its being penetrated by numerous vessels. 



The Corpus Albicans is a pea-like, white nodule placed on the middle 

 line, about the centre of the interpeduncular space. As will subse- 

 quently be learned, the body is formed by the reflection of the anterior 

 pillars of the fornix. 



The Tuber Cinereum is a layer of grey matter between the corpus 

 albicans and the optic commissure. It is perforated in its centre, and 

 connected to the upper surface of the pituitary gland by a hollow tube 

 of grey matter — the infundihuhmi. 



The Pituitary Body is a reddish-yellow, disc-shaped body, having a 

 diameter about equal to that of a sixpence. It is thickest in its centre 

 and thinnest at its rim. Its lower face rests on the sella turcica of the 

 sphenoid bone ; and its upper face receives the insertion of the infundi- 

 bulum, and covers the tuber cinereum, and, in part, the corpus albicans 

 and optic commissure. Within its striicture it comprises cells resembling 

 thos^ of the blood-vascular or ductless glands, and others that resemble 

 nei"vte cells. In the foetus it is proportionally larger, and contains a 

 cavity which communicates with the 3rd ventricle through the infundi- 

 bulum. 



The pons Tarini, corpus albicans, and tuber cinereum form the floor 

 of the 3rd ventricle, a cavity Avhich the dissector will hereafter expose 

 by working from the upper aspect of the cerebrum. 



The Lamina Cinerea, or Lamina Terminalis, is a thin, delicate layer 

 of grey matter which is placed above and in front of the optic commis- 

 sure. It is the anterior boundary of the 3rd ventricle. 



The Locus Perforatus Anticus is a spot of grey matter at each side 

 of the optic commissure, penetrated by numerous vessels for the corpus 

 striatum, which lies above the spot. 



The Fissure of Sylvius, is a faint and ill-defined groove Avhich begins 

 at the locus perforatus anticus, and extends outwards across the hemi- 

 sphere. 



The Great Longitudinal Fissure. In front of the optic chiasma the 



