40 THE MEDULLA OBLONGATA AND. PONS VAROL1I. 



nearly an inch (20 to 24 mm.), its greater breadth is about three-fourths of an inch 

 (17 to 18mm.) ; its thickness, from before backwards, is rather less (15 mm.). In 

 the lower part where it joins the spinal cord, its diameters differ but little from those 

 of the cord. The pons Varolii is about an inch long and rather more than an inch 

 broad ; it is considerably thicker than the medulla oblongata. 



The anterior and posterior median fissures which partly divide the spinal cord 

 are continued into similar fissures in the medulla oblongata. The anterior fissure 

 terminates immediately below the pons in a recess, the foramen cwcum of Vicq 

 d'Azyr ; it is partly interrupted below by the decussating bundles of the pyramids ; 

 the posterior fissure is continued upwards to about the middle of the bulb, it ceases 

 at the lower end of the fourth ventricle. 



The structure of the medulla oblongata and pons will be most easily made clear 

 by tracing the several parts of the spinal cord upwards. 



In doing this it will be found that the relative position and extent of many of 

 the parts are gradually altered, and that other parts which are not, so far as is known, 

 represented in the spinal cord, become interpolated between those which are there 

 met with. It will further be found that the change of relative position of the parts 

 is largely owing to two causes. In the first place the sudden passage of large 

 bundles of medullated fibres from the posterior pare of the lateral column of the cord 

 to the opposite side of the anterior median fissure appears to break up to a great extent 

 the grey matter of the anterior horn, which is traversed by the bundles. In the second 

 place the opening up of the central canal and separation of the lips of the posterior 

 median fissure bring the grey matter to the surface in the fourth ventricle, whilst the 

 posterior horn is coincidently shifted to the side, much in the same way as it would 

 be if a median incision were made from the posterior surface of the spinal cord into 

 the central canal, and the two lateral halves were then turned outwards so that the 

 sides of the posterior median fissure became the posterior surface of the cord. 



The lines along which the cranial nerve-roots issue from the sides of the bulb 

 mark the surface of the latter off into three chief columns on either side, which may 



Fig. 31. SECTION OF UPPER END OF SPINAL CORD AT THE JUNCTION 

 WITH THE MEDULLA OBLONGATA. (After Lockhart Clarke.) 



/, anterior; fp, posterior fissure ; p, end of decussation of pyramids ; 

 Cla, CIp, anterior and posterior roots of first cervical nerve ; XI, root 

 of spinal accessory nerve ; c, central canal. 



be termed dorsal, lateral, and ventral respectively. Thus 

 the spinal accessory or eleventh cranial nerve (figs. 32, 34, 

 35, XI) begins to take origin as far down as the lower 

 end of the cervical region of the cord where its roots issue 



from the side of the cord, and lateral to the posterior roots of the cervical nerves 

 (fig. 31). At the upper end of the cervical region, however, they are approached by 

 the line of the posterior roots, and some of their bundles arise in conjunction with 

 the posterior roots of the first and sometimes of the second cervical nerve. They are 

 succeeded by the bundles of the vagus root, and these again by those of the glosso- 

 pharyngeal. At the junction of the pons with the medulla oblongata the seventh 

 nerve also is seen issuing along the same line, and still higher the large root of the 

 fifth is given off from the lateral aspect of the pons. The line of exit of this 

 series of nerve-roots is marked below by a shallow sulcus, but if traced upwards 

 it will be seen that the sulcus is obliterated or nearly so before long, so that 

 the issuing series of nerve-roots alone serves to mark its position. As it passes 

 upwards it becomes gradually diverted outwards ; so that opposite the place where 

 the central canal opens into the calamus scriptorius of the fourth ventricle, the line 

 iu question has left the posterior surface and in the rest of its course runs along 



