CHAP. IL] THE BRAIN. 941 



This and Figs. 110 114, form a series of transverse dorsoventral sections of 

 the brain taken at different levels from th. ; hm 1 md of the bulb to the front of the 

 third ventricle ; the several levels are shewn by the lines drawn in Fig. 108. They 

 are all magnified twice. The details are shewn, for the sake of simplicity, in 

 diagrammatic fashion ; the white matter is left unshaded, the course of the fibres 

 being indicated in a few important instances only ; the grey matter is shaded 

 formally, the nerve-cells being indicated in the case only of the nuclei of the cranial 

 nerves. The want of complete bilateral symmetry which is often met with in such 

 sections is indicated in several of the figures. 



1. At the hind limit of the decussation of the pyramids. 2. In the middle of 

 the decussation. 3. At the upper end of the decussation. 4. Just below 

 the point of the calamus scriptorius. 5. Just above the point. 6. Through 

 the middle of the ala cinerea. 



Py. Pyramids. Py. dec. decussation of the pyramids. Supra Py. dec. superior 

 decussation. /. a. i. internal arcuate fibres. /. a. e. external arcuate fibres. 

 Cb. position of cerebellar tract. R. restiform body or inferior peduncle of the 

 cerebellum, e. p. external posterior column, fasciculus cuneatus, m. p. median 

 posterior column, fasciculus gracilis. r. raphe. 



/. h. lateral horn. in. p. n. nucleus of the median posterior column or gracile 

 nucleus, e. p. n. nucleus of the external posterior column or cuneate nucleus. 

 e. p. n. (m.) median division and e. p. n. (I.) lateral division of the same. 

 ol. olivary body. ol. a. median accessory, and ol. e. lateral accessory olive. 

 in. ol. interolivary layer, a. I. n. lateral (antero-lateral) nucleus, n. a. arcuate 

 nucleus, a. c. remnant of anterior horn. /. ret. reticular formation. . g. 

 substance of Rolando. 



a. r. c. I. anterior root, and p. r. c. I. posterior root of first cervical nerve. XI. root 

 of spinal accessory nerve. XII. twelfth or hypoglossal nerve, n. XII. nucleus 

 of the same in 6 ; the nucleus may be traced however through 2, 3, 4, 5, in 

 connection with the fibres of the nerve. . X. sensory or main part of the 

 glossopharyngeal-vago-accessory nucleus. X. m. motor nucleus of the vagus, 

 or nucleus ambiguus. IX. a. ascending root of the glossopharyngeal nucleus. 

 V. a. ascending root of the fifth nerve. 



4th. fourth ventricle ; the ependyma or lining is indicated by a thick dark line ; 

 and in 5 and 6, the tooth-like section of the projecting obex is shewn. 



In a section a little higher up (Fig. 109, 2), these decussating 

 fibres form on each side a large strand which starts from a part 

 of the anterior column, now becoming distinctly marked off as 

 the pyramid (Py.), and is apparently lost in the reticular forma- 

 tion, but in reality passes on to the crossed pyramidal tract of 

 the lateral column. This strand, as it crosses over, completely 

 cuts off the head of the anterior horn from the more central 

 grey matter, and forms with its fellow a large area of decussating 

 fibres between the bottom of the anterior fissure and the central 

 grey matter. When a surface view of the bulb is examined the 

 decussation is seen to be effected by alternate bundles, passing 

 now from right to left, now from left to right ; and in transverse 

 sections we find correspondingly that the anterior fissure appears 

 bent now to the left and now to the right, according as the 

 section cuts through a bundle passing from left to right or from 

 right to left. 



In sections still higher up (Fig. 109, 3 and 4) this conspicuous 

 strand of fibres crossing obliquely from side to side, will be no 

 longer seen ; decussating fibres are seen dorsal to the anterior 

 fissure, but these, of which we shall speak presently, are of 



