594 THE BRAIN. 



matter of the other side, and so may be regarded as part of the whole pyram- 

 idal tract ; but others are of different origin ; and even in the region of 

 the actual decussation of the pyramids some of the fibres which cross over 

 do not belong to the pyramidal tract. This system of decussating fibres 

 becomes increasingly prominent above the decussation of the pyramids, and 

 through it the ventral area of the bulb between the central canal and the 

 anterior fissure is much increased. The fibres as they cross form a middle 

 line of partition, the raphe (Fig. 132, 4, 5, r) which increases in depth in the 

 upper parts of the bulb, and on each side of the raphe help to break up the 

 gray matter (which previously formed the anterior horns) into what is 

 called the reticular formation. We shall return to this presently, but may 

 here call attention to a special development of these decussating fibres 

 which is seen just above the decussation of the pyramids. In a section at 

 this level (Fig. 132, 3) a strand of fibres (Supra Py. dec.} may be seen to 

 start chiefly from the gracile nucleus but also to some extent from the cune- 

 ate nucleus, to sweep round the central gray matter, and to decussate ven- 

 tral to this between it and the bottom of the anterior fissure. This is called 

 the superior decussation, or, for reasons which we shall see later on, the sen- 

 sory decussation. 



520. We must now turn to the posterior fissure and its relations to 

 the fourth ventricle. We saw that at the beginning of the pyramidal de- 

 cussation, the posterior horns had been thrown backward and outward so as 

 to increase the posterior columns. The posterior fissure is still of great 

 depth, so that by the increase of depth and maintenance of depth the poste- 

 rior column, the lateral limit of which is still sharply marked out by the 

 swollen head of the posterior horn as well as by the highest posterior root- 

 lets of the first cervical nerve, acquires at this level its maximum of bulk. 



From this point forward the depth of the posterior fissure and the dorso- 

 ventral diameter of the posterior columns diminishes. The head of the 

 horn (Fig. 132, 2) is thrown still further outward into the lateral regions; 

 developments of gray matter at the base and to some extent at the neck of 

 the horn (of these we shall speak presently) encroach (Fig. 132, 3) dorsally 

 on the white matter of the columns ; and the central gray matter appears 

 to rise dorsally at the expense of the posterior fissure, in coincidence with 

 the development described above as taking place on the ventral side of the 

 canal. 



Still a little further forward, in a section, for instance (Fig. 132, 4), a 

 little way behind the apex of the calamus scriptorius, the central gray matter, 

 which still forms a rounded mass around the central canal, is brought yet 

 nearer to the posterior fissure. 



In a section yet a little further forward (Fig. 132, 5) carried through the 

 hinder narrower part of the fourth ventricle itself, it is seen that the central 

 canal has opened out on the dorsal surface, and that the gray matter, which 



form body or inferior peduncle of the cerebellum ; e. p. external posterior column, fasciculus 

 cuneatus ; m. p. median posterior column, fasciculus gracilis; r. raphe ; 1. h. lateral horn ; m. p. n. 

 nucleus of the median posterior column or gracile nucleus; e.p. n. nucleus of the external pos- 

 terior 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. inter- 

 olivary layer; a. 1. n. lateral (antero-lateral) nucleus; n. a. arcuate nucleus; a. c. remnant of an- 

 terior horn ; /. ret. reticular formation ; s. g. substance of Rolando ; o. 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 hy- 

 poglossal 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 ; s. X. sensory or main part of the glosso- 

 pharyngeal-vago-accessory nucleus; X. m. motor nucleus of the vagus, or nucleus ambiguus; IX. 

 . ascending root of the glosso-pharyngeal 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 shown. 



