ANATOMICAL DESCRIPTION 



289 



The gray matter of the medulla oblongata is partly 

 continuous with that of the cord, but is partly broken up 

 into independent groups of cells, or nuclei, which are the 

 deep origins of most of the cranial nerves. 



426. The Ventricles of the Brain, and the Cerebro-spinal 

 Fluid. The ventricles are irregular cavities in the brain, 

 communicating with one another and continuous with the 

 canal of the spinal cord (Fig. 133). The lateral ventricles 

 lie one in each hemisphere of the 



cerebrum. They open into the 

 third ventricle, which is in the mid- 

 dle line. A narrow canal, called 

 the aqueduct of Sylvius, connects 

 the third with the fourth ventricle, 

 of which the back of the pons and 

 the medulla oblongata form the 

 floor, while the overhanging cere- 

 bellum forms part of its roof. A 

 small opening in the pia mater, 

 which completes the roof of the 

 fourth ventricle, makes connec- 

 tion between these interior cavities and the external sur- 

 face o*f brain and cord, so that the fluid which fills the 

 space beneath the arachnoid is continuous with that in 

 the inside. 



The cerebro-spinal fluid is a thin, watery substance which 

 bathes the external surfaces of the brain and spinal cord, 

 and fills the ventricles and the spinal canal. 



427. The Cranial, or Cerebral, Nerves (Figs. 134 and 135). 

 Of the twelve pairs of nerves issuing from the brain, 

 ten have their deep origins in the floor of the fourth ven- 

 tricle or in adjacent gray matter. Each cranial nerve is 

 said to have a deep origin, which is that portion of gray 



Fig. 133. Cast of brain 

 cavities. 



