694 



THE NERVOUS SYSTEM. 



Middle 



'peduncle of 



cerebellum. 



FIG. 369. Medulla oblongata and pons 

 Varolii. Anterior surface. 



in shape, its broad extremity directed upward, its lower end being narrow at its 

 point of connection with the cord. It measures an inch in length, three-quarters 

 of an inch in breadth at its widest part, and half an inch in thickness. Its surface 

 is marked, in the median line, in front and behind, by an anterior and a posterior 



median fissure, which are continuous with 

 similar fissures on the anterior and posterior 

 surfaces of the cord. The anterior fissure 

 contains a fold of pia mater, and terminates 

 just below the pons in a cul-de-sac, the fora- 

 men caecum of Vicq d'Azyr. It is inter- 

 rupted at its lower part by some bundles of 

 fibres, which cross obliquely from one side 

 to the other, forming the decussation of the 

 pyramids. The posterior is a deep but nar- 

 row fissure, continued upward to about the 

 middle of the medulla, where it expands into 

 the fourth ventricle. 



These two fissures divide the medulla into 

 two symmetrical halves, each half presenting 

 elongated eminences, which are continuous 

 with the columns of the cord. By taking 

 the lines along which some of the cranial 

 nerves emerge from the medulla, as land- 

 marks, the surface of this portion of the 

 nervous system may be divided into three columns, in the same way as the spinal 

 cord is divided into three columns by the lines corresponding to the points of exit of 

 the anterior and posterior roots of the spinal nerves. The anterior column com- 

 prises that portion Avhich is situated between the anterior median fissure and the 

 fibres of origin of the hypoglossal nerve : this column is called the pyramid. The 

 lateral column comprises that portion which is situated between the fibres of origin 

 of the hypoglossal nerve and the fibres of origin of the glosso-pharyngeal, pneu- 

 mogastric, and spinal accessory nerves. In the lower part of the medulla this 

 column is single, and is called the lateral tract ; but in the upper part an oval- 

 shaped body comes forward between it and the pyramid, and pushes aside the 

 lateral tract. This is called the olivary body. The posterior column comprises 

 that portion which is situated between the fibres of the origin of the glosso-pharyn- 



feal, pneumogastric, and spinal accessory nerves and the posterior median fissure, 

 t is marked by slight furrows dividing it into smaller columns, and these in the 

 lower part of the medulla are named, from without inward, the funiculus of 

 Rolando, the funiculus cuneatus, and the funiculus gracilis ; in the upper part 

 of the medulla, the funiculus of Rolando and the funiculus cuneatus appear 

 to become fused together, forming a single bodv, called the restiform body 

 (Fig. 370). 



The pyramids are two pyramidal bundles of white matter, placed one on either 

 side of the anterior median fissure, and separated from the olivary body by a slight 

 depression, from which the roots of the hypoglossal nerve emerge. At the lower 

 border of the pons these bodies are somewhat constricted and are here crossed 

 by a band of arched fibres, the ponticulus of Arnold ; below this they become 

 enlarged, and then taper as they descend to their lower extremity. The fibros of 

 which these pyramids are composed may be arranged in two bundles : an outer, 

 continuous below with the direct pyramidal tract of the anterior column of the 

 same side of the spinal cord, and an inner, continuous with the crossed pyramidal 

 tract of the lateral column of the opposite side of the cord. As will be subse- 

 quently mentioned, the direct pyramidal tract in the cord lies next to the anterior 

 median fissure, but as the crossed pyramidal tract of the cord ascends to the 

 medulla it decussates with its fellow of the opposite side across the anterior median 

 fissure, and so displaces laterally the direct pyramidal tract, and ascends, after decus- 



