366 SPECIAL ANATOMY. 



inferior concave surface forms upon either side the roof of the la- 

 teral ventricle, is situated in the middle line, in front, above the 

 septum ; behind, above the fornix. Its posterior thick and free 

 extremity (spknium) with the lyra s. psalterium, that is, the in- 

 ferior transverse fibres, covers the corpp. quadrigemina and coro- 

 narium, and touches the monticulus cerebelli. Its anterior extre- 

 mity is reflected upon itself (genu), and passes from above down- 

 wards and from before backwards, terminating, in a beaked man- 

 ner (rostrum), before the commissura anterior at the basis of the 

 cerebrum. 



2. Septum pellucidum, the translucent partition, a thin, trian- 

 gular, vertical plate higher before than behind, expanded between 

 the anterior part of corp. callosum and Iheforniz, consists of two 

 layers (between which the canal, septi pellucidi [fifth ventricle]), 

 and forms the internal walls of the lateral ventricle. 



3. Fornix, a )( shaped medullary arch, passing in the direction 

 of the corp. callosum, consists of two laminae which, beneath the 

 septum, are placed close to one another, between and upon the 

 optic tubercles, and before and behind separate from each other. 

 Its anterior crura (columnce) incline downwards and forwards 

 around the optic tubercles, in consequence of which a semilunar 

 space, foramen, Monroi, arises (through which the lateral com- 

 municates with the third ventricle). They then curve between 

 thalamus and corpus striatum to the corp. mammillaria, and, 

 from these, may be traced, as far as Uenia semicircularis and 

 corpp. quadrigemina, backwards, to the optic tubercles. Its pos- 

 terior crura are more widely separated from one another (between 

 them the lyra), and curve round the posterior surface of the tha- 

 lamus outwards and downwards to the pedes Hippocampi in the 

 lateral ventricles. 



4. Conarium s. glandula pinealis, a conical reddish-gray cor- 

 puscle (four lines long, two to three lines broad), between the two 

 anterior of the corpp. quadrigemina, invested with Pia mater, 

 surrounded by Vv. Galeni,\s situated behind the commis. poste- 

 rior of the third ventricle, below the lyra, its free apex directed 

 backwards. It is connected by means of two crura with the optic 

 tubercles, is sometimes hollow, and contains the yellow and white 

 sand of the brain (phosphate and carbonate of lime). 



5. Corpora quadrigemina s. bigemina (lobus opticus of the lower 

 animals), are the two larger anterior (nates) and the two smaller 

 posterior (testes} elevations which are situated in a quadrangle of 

 ten lines long and eight broad at the superior extremity of the 

 crura cerebri above the commencement of aquceduct. Sylvii (hence 



