378 THE NERVOUS SYSTEM. 



at the sides, where they turn backward, they form the "middle peduncles 

 of the cerebellum." 



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 remaining longitudinal bundles of the medulla oblongata behind, 

 pass into and through the tuber annulare, between various irregularly 

 diffused deposits of gray substance. From the upper border of the 

 tuber annulare, they emerge in the form of two obliquely diverging 

 bundles of nerve fibres, the crura cerebri, or peduncles of the brain. 

 They are joined posteriorly by other longitudinal bundles coming from 

 the cerebellum, known as the " anterior peduncles of the cerebellum," 

 and forming the tracts of communication between the cerebellum and 

 the cerebrum. The crura cerebri then pass into the base of the brain, 

 thus completing its connection with the spinal cord. 



The structure of the cerebro-spinal axis, as a whole, may be described 

 as follows : There is a continuous tract of gray substance, surrounding 

 the central canal of the spinal cord, expanding into a superficial layer 

 on the floor of the fourth ventricle, and thence extending forward 

 around the aqueduct of Sylvius and on the vertical sides of the third 

 ventricle, until it terminates at the infundibulum. This is the " gray 

 substance of the medullary canal," and from it all the nerves of volun- 

 tary motion and general sensibility take their origin. Near its upper 

 part there are various additional deposits of gray substance, the largest 

 of which is the cerebellum ; and beyond its upper extremity is the 

 cerebrum, containing the cerebral ganglia at its base, and expanding 

 above into the convolutions of the hemispheres. The two hemi- 

 spheres are connected with each other by a broad transverse commis- 

 sure of white substance, the "corpus callosum," covering the lateral 

 ventricles and the cerebral ganglia ; and the two lateral halves of 

 the cerebellum are united in a similar way by the pons Varolii. 



The longitudinal connections of the cerebro-spinal axis are the con- 

 tinuations of the columns of the cord. On emerging from the tuber 

 annulare, under the form of the crura cerebri, they enter the base of the 

 brain, and meet at once with the gray substance of the cerebral ganglia. 

 They pass through and between these ganglia, forming in this situation 

 the so-called " internal capsule;" and from its upper border they spread 

 out on each side into an expanding crown of divergent fibres, known 

 as the "corona radiata." The fibres of the corona radiata thence dis- 

 perse in every direction, to reach, at last, the gray matter of the cere- 

 bral convolutions. 



It is by no means certain that the individual nerve fibres are con- 

 tinuous throughout these longitudinal connections. On the contrary, 

 the study of successive microscopic sections, by the best observers, has 

 failed to show such a direct continuity ; and it is considered more prob- 

 able that the fibres coming from one portion of the cerebro-spinal axis 

 terminate in certain deposits of gray substance, and that the connec- 



