STRUCTURE OF THE MEDULLA OBLONGATA. 227 



LESSONS XXXIX. AND XL. 



TEE MEDULLA OBLONGATA, PONS, AND MESENCEPHALON. 



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

 of the pyramids, (b) just above the decussation, (c) opposite the middle of the 

 olivary body, and (d] either through the uppermost part of the olivary body, 

 or just above it. 



2. Section through the middle of the pons Varolii. 



3. Sections across the region of the corpora quadrigemina, 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 or bulb 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. 257) has much the same form as a section through the upper part 



FIG. 257. SECTION OF THE MEDULLA OBLONGATA 

 AT THE MIDDLE OF THE DECUSSATION OF THE 

 PYRAMIDS. (Lockhart-Clarke.) f. 



/, anterior, f.p., posterior fissure ; a.p. t pyramid ; , 

 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, con- 

 tinuation of substantia gelatinosa of Rolando ; p.c. , 

 continuation of posterior cornu of grey matter ; 

 f .g., funiculus gracilis. 



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 



