THE STRUCTURE OF THE MEDULLA OBLONGATA 



133 



4. The sensory nuclei (Fig. 92, 4), within which the afferent fibers terminate, 

 contain the cells of origin of the sensory fibers of the second order (Fig. 92, 2). 

 Some of these are short; others are long, and these may be either direct or 

 crossed. Many of them divide into ascending and descending branches. They 

 run in the reticular formation and some of the ascending fibers reach the thal- 

 amus. 



5. These sensory fibers of the second order give off collaterals to the motor 

 nuclei. Direct collaterals from the sensory fibers of the cranial nerves to the 

 motor nuclei are few in number or entirely wanting. 



6. The motor nuclei (Fig. 92, 5) are aggregations of multipolar cells which 

 give origin to the motor fibers of the cranial nerves (Fig. 92, 3). 



Main sensory nucleus 

 of trige minal nerve 



Afferent fiber of 

 second order 



Tractus solilarius 



N2(deus of 



hypoglossal nerve 

 Afferent fiber of 



second order 

 Spinal tract of 



trigeminal nerve 



and Us nucleus 



Fig. 92. Diagram of the tongue and rhombencephalon to illustrate the central connections 

 and functional relationships of certain of the cranial nerves: 1, Sensory neurons of the first order 

 of the trigeminal and glossopharyngeal nerves; 2, sensory neurons of the second order; 3, motor 

 fibers of the hypoglossal nerve; 4, sensory nuclei; 5, motor nucleus of hypoglossal nerve. (Cajal.) 



The Rearrangement Within the Medulla Oblongata of the Structures Con- 

 tinued Upward from the Spinal Cord. At the level of the rostral border of the 

 first cervical nerve the spinal cord goes over without a sharp line of demarcation 

 into the medulla oblongata. The transition is gradual both as to external form 

 and internal structure; but in the caudal part of the medulla there occurs a 

 gradual rearrangement of the fiber tracts and alterations in the shape of the 

 gray matter, until at the level of the olive, a section of the medulla bears no 

 resemblance to one through the spinal cord. 



The realignment of the corticospinal tracts and the termination of the fibers 

 of the posterior funiculi of the spinal cord are two of the most important factors 



