THE NERVOUS SYSTEM 475 



prominent a feature in man, but in the substance of the medulla 

 well-marked olivary nuclei exist. The pyramids are formed by 

 the collection of motor fibres descending from the brain, and 

 brought together in two bundles in the bulb. In the pyramids 

 they decussate, so that fibres from the right brain pass down the 

 left side of the cord, and vice versa. It is more convenient to 

 build up the bulb from below, so that, though the pyramidal 

 fibres are passing out of the brain, they are, from a constructional 

 point of view, spoken of as if they were passing the other way. 

 This being so, it is usual to describe the pyramidal fibres in the 

 cord as crossing in the medulla, and by so doing cutting through 

 the ventral horns of the grey matter, and eventually causing 

 them to disappear, as may be seen from sections taken at a 

 higher level. In this way the pyramids are formed, and little is 

 left of the original grey matter. On the dorsal surface of the 

 bulb the two horns open out ; the tip of each swells, and forms 

 the substantia gelatinosa, while the columns of Goll and Burdach, 

 which, it will be remembered, have their fibres passing into the 

 brain, end in nuclei known as the nucleus gracilis and nucleus 

 cuneatus respectively. These are often called the ' leg ' nucleus 

 and ' arm ' nucleus. The fibres from the above columns ter- 

 minate by arborising around the cells in these nuclei ; those 

 from the anterior half of the body terminate in the nucleus 

 cuneatus, while those from the posterior half terminate in the 

 nucleus gracilis. From the cells in these nuclei fresh axons are 

 formed, which pass forward into the higher centres, and so still 

 further increase the complicated arrangement of the bulb. 

 Incidentally attention may be drawn to the fact that this is the 

 second cell-station on the sensory path, the first being the 

 ganglia on the dorsal roots of the spinal nerves. The fibres 

 passing forward into the higher centres mentioned above form 

 the internal arcuate fibres, and in their journey fonvards de- 

 cussate above the pyramids, and continue their course through 

 the bulb and pons under the name of the fillet, and so reach the 

 optic thalamus ; here, for the third time, the fibres arborise 

 around cells. The fillet, or lemniscus, is consequently the sensory 

 path connecting the between-brain and the body. 



These changes in the arrangement of the medulla having brought 

 the central canal of the cord close to the dorsal surface, it soon 

 opens out into the fourth ventricle ; and now a further com- 

 plexity in the arrangement of the bulb is evident from the iact 

 that the nuclei giving origin to the"^cranial nerves enter the 

 field, those of the tenth and twelfth Jpairs being seen forming 

 the grey matter of the floor of the"\fourth ventricle. A fresh 

 mass of grey matter also appears, known as the olivary body, 

 while further internal arcuate fibres cross to the opposite side, 



