THE NERVOUS SYSTEM. 



113 



CRURA CEREBRI. 



Structure. The crura cerebri (in, Fig. 326), are principally formed 

 of nerve-fibres, of which the inferior or more superficial (crusta) are con- 

 tinuous with those of the anterior pyramidal tracts of the medulla oblon- 

 gata, and the superior or deeper fibres (tegmentum) with the lateral and 

 posterior pyramidal tracts, and with the olivary fasciculus. Besides these 

 fibres from the medulla oblongata, are others from the cerebellum; and 



FIG. 327. Dissection of brain, from above, exposing the lateral fourth and fifth ventricles with 

 the surrounding parts. % a, anterior part, or genu of corpus callosum; 6; corpus striatum; b', the 

 corpus striatum of left side, dissected so as to expose its grey substance ; c, points by a line to the 

 taenia semicircularis; d, optic thalamus; e, anterior pillars of fornix divided; below they are seen 

 descending in front of the third ventricle, and between them is seen part of the anterior commissure ; 

 in front of the letter e is seen the slit-like fifth ventricle, between the two laminae of the septum luci- 

 dum ; /, soft or middle commissure ; (/, is placed in the posterior part of the third ventricle ; immedi- 

 ately behind the latter are the posterior commissure (just visible) and the pineal gland, the two crura 

 of which extend forward along the inner and upper margins of the optic thalami ; h and z, the cor- 

 pora quadrigemina; fc, superior cms of cerebellum; close to fc is the valve of Vieussens. which has 

 been divided so as to expose the fourth ventricle; I, hippocampus major and corpus fimbriatum, or 

 taenia hippocampi; m, hippocampus minor; n, eminentia collaterals ; o, fourth ventricle; p, posterior 

 surface of medulla oblongata; r, section of cerebellum; s, upper part of left hemisphere or cerebel- 

 lum exposed by the removal of part of the posterior cerebral lobe. (Hirschfeld and Leveille.) 



some of the latter as well as a part of the fibres derived from the lateral 

 tract of the medulla oblongata, decussate across the middle line. 



Each cms cerebri contains among its fibres a mass of grey substance, 

 the locus niger. 



Functions. With regard to their functions, the crura cerebri may 

 be regarded as, principally, conducting organs: the crusta conducting 

 VOL, II. 8 



