580 



THE CRANIAL NERVES. 



Fig. 183. 



central in its position; what corre- 

 sponds to the posterior horns of 

 gray matter in the cord spreading 

 out lateral!}', and what corresponds 

 to the anterior horns following the 

 central canal as it recedes, and at 

 last occupying the middle of the 

 floor of the fourth ventricle, next the 

 median line. All the sensitive and 

 motor cranial nerves take their origin 

 from this layer of gray matter, or its 

 continuation, from the commence- 

 ment of the fourth ventricle to the 

 aqueduct of Sylvius beneath the tu- 

 bercula quadrigemina. The relations 

 of origin between the motor and sen- 

 sitive nerve roots are still preserved. 

 In the spinal cord, the motor roots 

 originate from the anterior horns of 

 gray matter, the sensitive roots from 

 the posterior horns. In the medulla 

 oblongata and tuber annulare, the 

 nuclei of the motor cranial nerves 

 form a series near the median line; 

 those of the sensitive nerves are 

 placed farther outward. A series of 

 sections of the spinal cord, medulla 

 oblongata, and tuber annulare, made 

 in succession from below upward, 

 show that the collections of gray 

 matter, or nuclei, in the medulla 

 oblongata and tuber anuulare, from 

 which the different motor or sensitive 

 nerves take their origin, are not com- 

 pletely disconnected from each other 

 any more than the successive portions 

 of gray matter in the spinal cord; 



I. Transverse Section of the Tuber Annulare, through the lower border of the pons Varolii. 

 1. Nucleus of the facial and abducens nerves. 2. Nucleus of the auditory nerve. F. Facial 

 nerve. Ab. Abducens nerve. 



II. Transverse section of the medulla oblonjata, through the middle of the olivary bodies, 

 and just above the opening of the central canal. 1. Hypoglossal nucleus. 2 Pneumogastric 

 nucleus. H. Hypoglossal nerve. Pn. Pneumogastric nerve. 



III. Transverse section of the medulla, through the lower end of the olivary bodies, and 

 just below the opening of the central canal. 1. Hypocrlossal nucleus. 2. Pneumogastiic 

 nucleus. H. Hypoglossal nerve. Pn. Pneumogastric nerve. 



IV. Transverse section of the medulla, through the decussation of the anterior pyramids. 

 Sp. Spinal accessory nerve. 



V. Transverse section of the spinal cord in the dorsal region, a, a. Anterior nerve roots. 

 p, p. Posterior nerve roots. 



