MEDULLA OBLONGATA. 385 



Sylvius. In their interior they contain grey matter, which has a semilunar 

 shape when the crus is divided transversely, and has been termed the locus. 

 niger. The third nerve will be observed to arise from the inner side of 

 each, and the fourth nerves wind around their outer border from above. 



The Pons Varolii* (protuberantia annularis, nodus encephali), is the 

 broad transverse band of white fibres which arches like abridge across the 

 upper part of the medulla oblongata ; and, contracting on each side into a 

 thick rounded cord, enters the substance of the cerebellum under the name 

 of crus cerebelli. There is a groove along its middle which lodges the 

 basilar artery. The pons Varolii is the commissure of the cerebellum, and 

 associates the two lateral lobes in their common function. Resting against 

 the pons, near its posterior border, is the sixth pair of nerves. On the 

 anterior border of the crus cerebelli, at each side, is the thick bundle of 

 filaments belonging to the fifth nerve, and, lying against its posterior border, 

 the seventh pair of nerves. The upper surface of the pons forms a part of 

 the floor of the fourth ventricle. 



MEDULLA OBLONGATA. 



The medulla oblongata (bulbus rhachidicus), is the upper enlarged por- 

 tion of the spinal cord. It is somewhat conical in shape, and a little more 

 than an inch in length, extending from the pons Varolii to a point corre- 

 sponding with the upper border of the atlas. On the middle line, in front 

 and behind, the medulla oblongata is marked by two vertical fissures, the 

 fissura longitudinalis anterior and posterior, which divide it superficially 

 into two symmetrical lateral cords or columns; whilst each lateral column 

 is subdivided by minor grooves into three smaller cords, namely, the cor- 

 pora pyramidalia, corpora olivaria, and corpora restiformia. 



The Corpora pyramidalia are two narrow convex cords, tapering slightly 

 from above downwards, and situated one on either side of the fissura lon- 

 gitudinalis anterior. At about an inch below the pons the corpora pyra- 

 midalia communicate very freely across the fissure by a decussation of their 

 fibres, and at their point of entrance into the pons they are constricted into 

 round cords. The fissura longitudinalis is somewhat enlarged by this 

 constriction, and the enlarged space has received the name of foramen 

 caecum of the medulla oblongata. 



The Corpora olivaria (named from some resemblance to the shape of 

 an olive), are two oblong, oval-shaped, convex bodies, of about the same 

 breadth with the corpora pyramidalia, about half an inch in length, and 



tuber cinereum, from which the infundibulum is seen projecting. 12. The corpora al- 

 bicantia. 13. The locus perforatus, bounded on each side by the crura cerebri, and by 

 the third nerve. 14. The pons Yarolii. 15. The crus cerebelli of one side-. 16. Thr> 

 fifth nerve emerging from the anterior border of the crus cerebelli; the. small nerv by 

 its side is the fourth. 17. The sixth pair of nerves. 18. The seventh pair of nerves con- 

 sisting of the auditory and facial. 19. The corpora pyramidalia of the medulla oblongrata; 

 the corpus olivare and part of the corpus restiforme are seen at each side. Just below 

 the numeral is the decussation of the fibres of the corpora pyramidalia. 20. The eighth 

 pair of nerves. 21. The ninth or hypoglossal nerve. 22. The anterior root of the first 

 cervical spinal nerve. 



* Constant Varolius, Professor of Anatomy in Bologna: died in 1578. He dissected 

 the brain in the course of its fibres, beginning from the medulla oblongata; a plan whict 

 has since been perfected by Vieussens, and by Gall and Spurzheim. The work con 

 taining his mode of dissection, " De Resolutione Corporis Humani," was published after 

 Ms death, in 1591. 



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