192 



THE ESSENTIALS OF HISTOLOGY 



LESSON XXXVII. 



TEE MEDULLA OBLONGATA, PONS, AND MESENCEPHALON. 



1. SECTIONS of the medulla oblongata made, (a) at the level of the decussation 

 of the pyramids, (b) opposite the middle of the olivary body, and (c) just 

 above the olivary body. 



2. Section through the middle of the pons Varolii. 



3. Sections across the region of the corpora quadrigenaina, one at the level 

 of the inferior, the other at the level of the superior, pair. 



In all the above sections sketch under a low power the general arrange- 

 ment of the grey and white matter, inserting the positions of the chief groups 

 of nerve-cells. 



[The tissue is hardened and the sections are prepared, stained, and 

 mounted in the same way as the spinal cord.] 



The structure of the medulla oblongata can best be made out by 

 the study of a series of sections taken from below upwards, and by 

 tracing in these the changes which occur in the constituent parts of 

 the spinal cord, taking note at the same time of any parts which may 

 be superadded. 



A section through the region of the decussation of the pyramids 

 (fig. 226) has much the same form as a section through the upper part 



FIG. 226. SECTION OF THE ME- 

 DULLA OBLONGATA AT THE 

 MIDDLE OF THE DECUSSATION 

 OF THE PYRAMIDS. 



anterior, /. p. posterior fissure ; 

 a.p. pyramid ; a, remains of part of 

 anterior cornu, separated by the 

 crossing bundles from the rest of the 

 grey matter; I, continuation of 

 lateral column of cord ; R, continua- 

 tion of substantia gelatinosa of Ro- 

 lando ; p.c. continuation of posterior 

 icornu of grey matter ; f.g. funiculus 

 pracilis. 



a.c 



of the spinal cord, and most of the structures of the cord can be easily 

 recognised. A considerable alteration of the grey matter is, however, 

 produced by the passage of the large bundles of the crossed pyramidal 



