OF THE NERVOUS SYSTEM. 95 



is composed. It is connected with the medulla spinalis, 

 the pons, and cerebrum by peduncles of medullary fibres 

 called crura. In volume it is about one-third of the cere- 

 brum, and is larger comparatively in the child than in the 

 adult. 



188. Optic Lobes. By removing the upper portion of 

 the hemispheres and the corpus callosum, the ventricles of the 

 brain are exposed ; also certain rounded masses, forming as it 

 were the base of the section. These masses, named in suc- 

 cession from before backwards, are the corpora striata, the 

 thalami nervorum opticorum, and the tubercula quadri- 

 gemina; and on the back and somewhat lower part of the 

 thalami may be seen, by raising the thalami upward, certain 

 rounded elevations of a greyish colour ; these are the corpora 

 geniculata. On all these structures important experiments 

 have been made in living animals. 



189. Spinal Marrow; Medulla Spinalis. The 

 medulla spinalis (Fig. 55 c ; Fig. 57/), may be viewed as a 

 continuation of the medulla oblongata and that of the brain 

 itself. A median fissure of no great depth divides it into 

 two lateral portions, anteriorly and posteriorly. Its upper 

 extremity, which is enlarged, is usually called the bulb, by 

 others the medulla oblongata : here are to be seen various 

 swellings called olivary, pyramidal, and restiform bodies ; and 

 at the sides of this medulla oblongata and of the spinal 

 marrow following it, may be seen proceeding outwards the 

 nerves. No nerves come from or go to the cerebrum proper, 

 unless it be the olfactory. Where the nerves leave the spinal 

 marrow to proceed to the pectoral extremities, the organ is 

 usually enlarged ; and the same occurs, but not so distinctly, 

 lower down, where the nerves leave to proceed to the pelvic 

 extremities. In man, the spinal marrow descends only as low 

 as the second lumbar vertebra, terminating in a fine thread- 

 like body, by which it is attached to the lower part of the 

 column. The inferior portion of the spinal canal, lumbar, 

 sacral, and coccygeal (where it is quite imperfect), is occu- 

 pied with the membranes and by the nerves proceeding from 

 the spinal marrow. This assemblage of nerves within the 

 canal was called by the older anatomists the cauda equina, 

 in the centre of which will be found the terminating filum 

 or thread of the organ itself. 



The spinal marrow is composed, like the brain and cere- 



