440 



GENERAL ARRANGEMENT OF 



Fig. 153. 



In its deeper parts, the tuber annulare contains longitudinal tracts of 

 white substance, passing upward from the medulla oblongata toward 

 the cerebrum. The continuation of the anterior pyramids in front, and 

 the remainder of the longitudinal bundles of the medulla oblongata 

 behind, pass into and through the substance of the tuber annulare, where 

 they are mingled with an irregular diffused deposit of gray substance. 



From the upper border of the tuber annu- 

 lare, the longitudinal tracts of white sub- 

 stance emerge in the form of two thick, 

 obliquely diverging, bundles of nerve fibres, 

 the crura cerebri, or peduncles of the brain. 

 They are joined posteriorly by other longi- 

 tudinal bundles coming from the cerebellum, 

 known as the "anterior peduncles of the 

 cerebellum*" which are the organs of com- 

 munication between the cerebellum and the 

 cerebrum. The fibres of the crura cerebri, 

 thus constituted, then plunge into the sub- 

 stance of the two collections of gray matter 

 known as the " cerebral ganglia," namely, 

 the corpora striata and optic thalami ; thus 

 making a connection between these ganglia 

 and the medulla oblongata and spinal cord 

 below. 



Finally, from the outer and upper por- 

 tions of the cerebral ganglia, the nerve fibres 

 of the white substance radiate in all direc- 

 tions, following a more or less curvilinear 

 course from within outward, until they reach 

 the gray substance of the convolutions at 

 the surface of the hemispheres. The cere- 

 bral convolutions of the two sides are also 

 united by the transverse fibres of the cor- 

 pus callosum. 



The entire brain may, therefore, be con- 

 sidered as a symmetrical series of nervous centres connected with each 

 other and with the spinal cord by longitudinal tracts of white substance. 

 They occur in the following order : 1. The olfactory lobes, of small size 

 and concealed beneath the anterior portion of the brain ; 2. The cerebral 

 hemispheres, surrounding and covering the remaining parts by their 

 lateral and posterior extension; 3. The corpora striata, 4. The optic 

 thalami, and 5. The tubercula quadrigemina, occupying the central por- 

 tion of the base of the cerebrum, and resting upon 6, the crura cerebri; 

 Y. The tuber annulare ; 8. The cerebellum, and 9. The medulla oblon- 

 gata. Of the collections of gray substance just enumerated, the cere- 

 brum and cerebellum only are convoluted externally, the others being 



MEDTTLLA OBLONGATA, 

 TUBER ANNTJLAEK, AND 

 CRURA CEREBRI. The super- 

 ficial and deep transverse fibres 

 of the tuber annulare have been 

 cut away, showing the continua- 

 tion of the longitudinal fibres in 

 its interior. 1. Decussation of the 

 optic nerves. 2. Crus cerebri. 3. 

 Lateral portion of the pons Va- 

 rolii. 4. Anterior pyramid. 5. Oli- 

 vary body. (Hirschfeld.) 



either smooth and rounded or irregular in form. 



