638 PHYSIOLOGY. 



pontal, and the external, or voluntary motor, bundle. The cortico- 

 pontal bundle acts as a commissure between the cerebrum and cere- 

 bellum. It passes from the anterior region of the cerebrum through 

 the peduncles to the pons and medulla, to end in the cerebellum. 

 The voluntary motor bundle descends from the motor regions of the 

 cortex to end in the nuclei of origin of the cranial and the spinal 

 nerves. 



TEGMENTUM. The superior layer of the cerebral peduncle, 

 known as the tegmentum, is chiefly the formatio reticularis and 

 fillet, which consists of masses of gray matter and fibers which ex- 

 tend through the posterior end of the medulla oblongata, pons/ and 

 crura up to the optic thalami. At the height of the corpora quad- 

 rigemina is a reddish column formed of multipolar cells. It is the 

 red nucleus of the tegmentum. 



THE Locus NIGER, which separates the pes, or crusta, from the 

 tegmentum, consists of highly pigmented cells. They are like the 

 cells of the motor regions of the cortex. Thus, the locus niger might 

 be considered as a sort of motor ganglion whose cells are charged 

 with black pigment. 



The Fourth Ventricle. 



The fourth ventricle is a rhomboid cavity (sinus rhomboidalis) 

 imbedded upon the posterior surface of the medulla oblongata and 

 pons. It is the space into which the central canal of the cord opens 

 out superiorly. It is flattened from top to base; and has an inferior 

 wall, or floor; a superior wall, or vault; and four angles. 



Floor of the Ventricle. The floor of the fourth ventricle is 

 lozenge-shaped, being formed by two triangles placed in contiguity 

 at their bases. It is lined by a layer of gray matter, which is but a 

 continuation of that of the cord. 



The inferior triangle (calamus scriptorius) belongs to the pos- 

 terior face of the medulla; the superior triangle to the posterior 

 face of the pons. 



Upon the median line of the floor there is a slight groove : the 

 handle of the calamus. On each side of this groove the surface of 

 the floor presents small, rounded, and elongated prominences. These 

 have been described at some length previously, so that now they will 

 be but mentioned. In the inferior triangle, from the handle of the 

 calamus to the restiform body, they are: (1) trigonum hypoglossi; 

 (2) ala cinerea, or trigonum vagi; (3) trigonum acustici. 



In the superior triangle, upon each side of the median groove 



