MEDULLA, PONS, AND MIDBRAI^. 287 



B. The Medulla, Pons, and Midbrain. 



The brain-stem, comprising the medulla, pons, and mid- 

 brain, is the pasasge-way between the cord, the cerebellum, and 

 the cerebrum ; and, at the same time, a great reflex centre 

 with its own nerves, both motor and sensory. It will be con- 

 sidered under four heads : (1) the tracts that connect the cord 

 with the cerebrum ; (2) the tracts that connect the cerebellum 

 with the cord and the brain ; (3) its reflex centres ; and (4) 

 its nerves. 



Group 1. Two tracts connect the cord with, the brain, a 

 sensory and a motor. The latter is called the pyramidal tract. 

 The sensory path contains a part of the ventral and lateral 

 columns of the cord and almost all of the dorsal columns. In 

 entering the medulla, some of the fibres of the lateral and 

 ventro-lateral columns of the cord curve a little dorsalward 

 and inward, to make two bands of fibres that pass upward in 

 the medulla on either side of the raphe. These two bands 

 are the sensory path, here called the interolivary bundle. 

 At the same time the central canal of the cord curves dorsal- 

 ward and opens into the fourth ventricle. The roof of the 

 fourth ventricle is at first a thin veil of tissue, but opposite 

 the pons it becomes the cerebellum (Fig. 215). By this 

 thinning out of the dorsal wall the dorsal columns of the cord 

 are pushed outward to end in the two nuclei that make the 

 prominences on the surface of the medulla just above the clava. 

 From these nuclei, which represent the spinal nerves, as well 

 as from all the sensory nuclei of the medulla and the pons, 

 fibres curve across the brain-stem, decussate in the raphe, and 

 enter the sensory tract. These fibres are called the internal 

 arcuate fibres. 



Throughout the medulla the two bands or sheets of fibres 

 forming the sensory path are parallel. In passing into the 

 pons, however, the ventral fibres spread out like a fan into a 

 horizontal sheet, which divides the pons into two parts, a 

 dorsal and a ventral. Here the bundle is called the medial 

 lemniscus. In entering the midbrain the sheet curves outward 



