CHAPTER XXVI 

 THE NERVOUS SYSTEM (Continued) 



D. THE CENTRAL PATHS OF THE CRANIAL NERYES 



THE course of the spinal nerves has already been sufficiently 

 described in tracing the course of the great motor and sensory 

 paths of the spinal cord. Each spinal segment was found to con- 

 tain the nuclei of a ventral or motor nerve root, whose centrifugal 

 peripheral neurones begin in its ventral horns, together with cer- 

 tain cell groups of the posterior horns and intermediate zone which 

 serve as sensory nuclei, receive the end arborizations of peripheral 

 sensory neurones whose nerve cells lie in the spinal ganglion of a 

 dorsal nerve root, and send their neuraxes cephalad in one of the 

 various centripetal tracts of the spinal cord. In the medulla these 

 centripetal paths unite in the fillet, by which, with the aid of neu- 

 rones of higher orders, the centripetal impulses are finally con- 

 veyed to the cerebral cortex. 



The course of the cranial nerves, while conforming quite closely 

 to the same general arrangement, presents slight deviations from 

 this type which result in the appearance of special nuclear groups 

 and centripetal pathways. These peculiarities warrant a brief out- 

 line of the central paths of the several cranial nerves. 



The twelfth or hypoglossal nerve (Figs. 383 and 384) contains 

 only centrifugal or " motor" fibres. This nerve takes its origin 

 in the large-celled hypoglossal nucleus, which is ovoid in shape, 

 and is situated in the medulla oblongata on either side of the me- 

 dian line and just dorsal to the posterior longitudinal fasciculus 

 (Fig. 383). The nucleus can be traced through the lower half of 

 the medulla, and if it is stained according to the method of Wei- 

 gert, it can be readily distinguished by the many coarse medullated 

 fibres which it contains. 



The central neurones of the hypoglossal nerve come down from 

 the cerebrum in the pyramidal tracts. Having arrived at the level 



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