392 ANATOMY FOR NURSES [Ch.^p. XIX 



the latter is a straight, fairly uniform canal of very small diameter, 

 the former is at some points very narrow, and at others much 

 widened out so as to form quite jrood-sized chambers, and these 

 chambers are called the ventricles of the brain. These ventricles 

 are filled with cerebro-spinal fluid, just as the canal of the cord is 

 like\vise filled with the same material. 



The ventricles are five in number. The most posterior is the 

 enlargement or expansion of the central canal, occupying the sub- 

 stance of the medulla oblongata, and is called the " fourth ven- 

 tricle." Leading forward from the anterior end of the fourth 

 ventricle, the calibre of the canal again narrows to a very small 

 diameter ; the tube, on reaching the brain substance uniting the 

 two halves of the cerebrum, again expands into a somewhat smaller 

 chamber, called the "third ventricle." The small canal already 

 mentioned as joining the third and fourth ventricles is known as 

 the aqueduct of Sylvius. 



Toward the forward end of the third ventricle there are noted 

 two small channels, the foramina of Monro, one on either side 

 leading in a direction forward, upward, and outward, each fora- 

 men leading into a very large ventricle occupying the centre 

 of its corresponding cerebral hemisphere, called the lateral 

 ventricles. 



The fifth ventricle is very small, lies between the two lateral 

 ventricles, and is not in communication with the other ventricles. 



The student will thus see that both the brain and spinal cord 

 are really hollow. In some portions, however (as the spinal 

 cord), the interior cavity is so minute and the walls so exceed- 

 ingly thick that the cavity is a negligible quantity, and the 

 mass can practically be considered as solid ; on the other hand, 

 in the case of the ventricles, especially the lateral ventricles, the 

 cavity is large enough to occupy an appreciable space, and may 

 become overdistended with cerebro-spinal fluid in certain con- 

 ditions of disease. 



On the whole, the cavity of the brain and cord occupies a more 

 or less central position, having its walls at any given point of 

 about equal thickness ; at certain points, however (the third 

 and fourth ventricles especially), the cavity approaches so close 

 to the surface that at these points one of its walls is thinned out 

 to only a microscopic thickness. 



