VENTRICLES OF BRAIN 303 



of the cord, and this is the explanation of paralysis of one side 

 of the body, following injuries of the other side of the brain. 



The posterior columns of the medulla contain sensory fibers 

 going upward to the brain, while the lateral tracts contain both 

 motor and sensory fibers (like the cord). Most of the sensory 

 fibers also cross at different levels. 



The medulla contains centers for the most important nerves of 

 the body respiratory, cardiac, vaso-motor, etc. 



The pons Varolii, or bridge of Varolius, is situated in front of 

 the medulla, below the cerebrum and cerebellum, and so named 

 because fibers run through it from all three of the other parts of 

 the brain, as though it were a bridge between them. It, also, is 

 white externally and gray within, and is not unlike the cord, 

 although in a still more modified form than the medulla. 



Two large nerve bundles are seen diverging from the anterior border of the 

 pons, the crura of the cerebrum (often called peduncles). They contain all of 

 the motor and sensory fibers of the cerebrum which pass between the pons 

 and the cord. The fibers to and from the cerebellum form peduncles of 

 the cerebellum (smaller in size). 



THE FIVE VENTRICLES OF THE BRAIN (Fig. 195) 



The five ventricles are different portions of one cavity, which is 

 continuous with the central canal of the spinal cord. The two 

 lateral ventricles have been mentioned. The third ventricle is 

 between them, and the fourth ventricle is behind the third, being 

 in the medulla and pons; some of the most important nuclei or 

 centers of the body are imbedded in the floor of the fourth ventricle. 



Each lateral ventricle communicates with the third through an 

 opening called the foramen of Munro; the third communicates with 

 the fourth through the aqueduct of the cerebrum (aqueduct of 

 Sylvius), a slender canal in the crura and pons; and the fourth 

 ends in the central canal of the cord. These spaces are therefore 

 continuous, and they contain cerebro-spinal fluid. 



The so-called fifth ventricle is not a portion of the general cavity not a 

 true ventricle. It is a narrow space in front of the third, having no opening 

 whatever. 



Clinical notes. Hydrocephalus is caused by an accumulation of fluid in 

 the ventricles, enlarging them and pressing upon the brain substance, and 



