386 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



small blood-vessels. It is the nourishing tissue of 

 the brain and clothes its entire surface, dipping 

 down to the bottom of fissures, and sending strands, 

 associated with blood-vessels, into the brain sub- 

 stance. The brain pia is more vascular than that of 

 the cord. 



The arachnoid is a web-like membrane between the 

 dura and the pia, but much nearer the dura. The 

 space beneath the dura is called the subdural space, 

 and is small. That between the arachnoid and pia 

 is larger and is called the subarachnoid space. The 

 former has a little serous fluid, and the latter is filled 

 with lymph and some cerebrospinal fluid. This 

 fluid reaches the external surface of the brain through 

 a small foramen or pore in the thin roof of the fourth 

 ventricle. Trabeculae intervene between all these 

 membranes. 



THE MEDULLA. 



The medulla is about one inch in length, and rep- 

 resents the portion of the brain next to the spinal 

 cord. Its lower extremity is at the lower margin of 

 the foramen magnum. From this point it passes 

 upward in nearly a vertical direction to its upper ex- 

 tremity at the lower border of the pons. Being the 

 nerve center for the large cranial nerves, the medulla 

 is the most vital part of the brain, and the best pro- 

 tected. Its lower portion resembles the cord, having 

 the same fissures and grooves. The upper portion is 

 expanded in such a manner as to bring its cavity or 

 fourth ventricle to the dorsal surface. This ex- 

 panding process has carried the dorsal tracts later- 



