Section 4. 



Bulb (Medulla Oblongata), Pons, and Mid-Brain. 



The change undergone by the spinal cord merging itself into 

 the bulb may be studied by successive sections of the latter. 

 Briefly it may be stated that the central canal of the cord widens 

 in order to become subsequently the fourth ventricle ; the sym- 

 metrical and regular arrangement of the grey and white matter 

 of the cord becomes broken up, not only in order that it may find 

 its way to its destination in the higher centres, but in the case 

 of those fibres which are passing out, that they may be collected 

 from the various outlying centres, and brought together in an 

 orderly manner in the smaller and more compact structure, the 

 medulla. But the medulla is not only a highway for nerve- fibres 

 passing in two opposite directions — it is also a cell -centre ; 

 and besides restarting all the tracts which terminate in the 

 bulb on their way up or down, it also gives origin to six of the 

 most important cranial nerves. The origin of these in the 

 medulla, in the various grey nuclei, is an additional complication 

 to the rapidly changing appearance presented by the organ 

 in successive sections from rear to front. Apart from the com- 

 plexity of the subject we are now entering on, which increases 

 as we approach the cerebrum, there is also the important fact 

 that in the large herbivora, with but few exceptions, the course 

 of the fibres and the collocation of cells, not only in the bulb, 

 but also in the other centres, are not known with any degree 

 of accuracy. 



There is now good reason to hope that the experiments of King* 

 and other observers on the larger herbivora, will be the means of 

 laying the foundation of exact physiological knowledge with regard 

 to their nervous system. 



The bulb in the horse is wider and more flattened than the 

 cord ; it is about 2 inches in length, and is wider anteriorly than 

 posteriorly ; it is considered to begin at the first cervical 

 nerve and end at the pons. In this short length of nervous 

 material functions of vital importance are carried out, through 

 the medium of those cranial nerves which take their origin from 

 this part. On the ventral surface of the medulla two well-marked 

 structures exist, known as the pyramids. There is no surface 

 enlargement indicating the presence of the olivary bodies, so 



* Op. cit. 

 474 



