ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 639 



and near the base of the triangle, are seen two rounded eminences: 

 (1) eminentia teres and (2) the locus cwruleus. 



The various eminences correspond to the origin of the cranial 

 nerves. Thus, in the locus cceruleus is located the origin of the 

 small root of the trigeminus; in the teres eminentia the common 

 origin of the facial and abducent; in the trigonum hypoglossi is 

 the origin of the hypoglossal nerve; in the ala cinerea, or trigonum 

 vagi, occurs the origin of the motor roots of the glosso-pharyngeal 

 nerves, pneumogastric, and spinal accessory; in the trigonum acus- 

 tici are found the fibers of the auditory and the sensory fibers of 

 the mixed nerves, glosso-pharyngeal, vagus, and spinal accessory. 

 The trigonum hypoglossi corresponds to the funiculus teres; the ala 

 cinerea to a depression: posterior fovea. 



At the level of the middle of the floor of the fourth ventricle 

 a variable number of striae go out from the median groove toward 

 the lateral angles. Here they converge somewhat and form, accord- 

 ing to some authors, the posterior root of the auditory nerve. The 

 striations constitute the larbce of the calamus. 



The gray matter of the spinal cord, when it penetrates into the 

 medulla, exposes itself upon the floor of the fourth ventricle. The 

 horns of the central gray column of the cord are found broken up 

 into many parts by the decussation of the pyramids and fillet. By 

 reason of this, the gray matter in the floor of the ventricle repre- 

 sents four irregular, discontinuous longitudinal columns; two are 

 central, with a superficial one on each side. These columns are pro- 

 duced by the bases and detached heads of the anterior and posterior 

 horns of the central gray column. From the anterior gray matter 

 proceed motor nerves; from the posterior gray matter spring sen- 

 sory nerves. 



The lateral boundaries of the ventricle are, in the lower half, 

 Lhe clavae of the funiculi graciles, the cuneati, and the restiform 

 lies. In its upper half the superior peduncles of the cerebellum 

 form the limits. 



Aqueduct of Sylvius. 



The aqueduct of Sylvius is a canal a centimeter and a half long. 

 [t is hollowed out beneath the corpora quadrigemina. By means of 

 this aqueduct the fourth ventricle communicates with the third. It 

 is derived from the middle cerebral vesicle. Its walls are formed 

 ibove by the valve of Vieussens, the corpora quadrigemina, and the 

 rhite, posterior commissure. Its base, or floor, is formed by the 



