CHAPTER IX 



THE STRUCTURE OF THE MEDULLA OBLONGATA 



THE medulla oblongata contains the nerve-cells and fiber tracts associated 

 with certain of the cranial nerves. These include the central mechanisms which 

 control the reflex activities of the tongue, pharynx, and larynx, and in part those 

 of the thoracic and abdominal viscera also. At the same time the ascending 

 and descending fiber tracts, which unite the spinal cord with higher nerve 

 centers, pass through the medulla oblongata. 



The central connections of the cranial nerves, except those of the first two 

 pairs, are located in the medulla oblongata and in the tegmental portions of the 

 pons and mesencephalon. In many respects they resemble the connections of 

 the spinal nerves within the spinal cord. The following general statements on 

 this topic, most of which are illustrated in Fig. 92, will help to elucidate the 

 structure of the brain stem. 



1. The cells of origin of the sensory fibers of the cranial nerves (Fig. 92, 1) 

 are found in ganglia which lie outside the cerebrospinal axis and are homologous 

 with the spinal ganglia. These are the semilunar ganglion of the trigeminal, 

 the geniculate ganglion of the facial, the superior and petrous ganglia of the 

 glossopharyngeal, the jugular and nodose ganglia of the vagus, the spiral gang- 

 lion of the cochlear, and the vestibular ganglion of the vestibular nerve. 



2. All of these sensory ganglia except the last two, the cells of which are 

 bipolar, are formed by unipolar cells, the axons of which divide dichotomously 

 into peripheral and central branches. The latter (or in the case of the acoustic 

 nerve the central processes of the bipolar cells) form the sensory nerve roots, 

 enter the brain stem and divide, each into a short ascending and a long descending 

 branch. These branches give off numerous collaterals, which with the terminal 

 branches end in gray masses known as sensory nuclei or nuclei of termination. 

 It is the descending branches of the sensory fibers of the trigeminal neroe which 

 form the spinal tract of that nerve illustrated in Figs. 92, 98, 99, 101. 



3. The ascending branch may be entirely wanting, as in the case of the sen- 

 sory fibers of the seventh, ninth, and tenth nerves, all of which bend caudally and 

 form a descending tract in the medulla oblongata, known as the tractus soli- 

 tarius (Figs. 92, 101, 103). 



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