112 PHYSIOLOGY AND TEMPERANCE. 



different parts of the brain, and according to the fulness of 

 the blood-vessels. 



9. Protection of the Brain. The provision made to 

 protect the soft yielding brain from concussion requires some 

 notice. We have spoken of the protection afforded by the 

 smooth, round skull, with its two tables thickened and 

 strengthened at the most exposed parts. Within the skull, 

 this jelly-like mass does not lie like a lump of jelly. It is 

 supported in several ways. The front part rests upon a 

 shelving of bone, the roof of the nasal cavity. The middle 

 lobes have each a snug little cavity of their own, and the back 

 part rests on the flooring of membrane. The upright parti- 

 tion prevents one hemisphere from pressing upon the other, 

 and is a supporting column. Within the brain substance 

 itself are a number of small cavities containing fluid, and 

 these cavities communicate with the outer spaces. By this 

 arrangement the pressure is equalized, and as the fluid natu- 

 rally tends towards the lowest parts, the brain in reality rests 

 upon a perfect water-bed. 



Complete as this is, it is not the whole of that all-wise 

 arrangement by which the nerve centres are protected. The 

 cavities of the brain communicate with the cavity of the 

 spinal canal, so that the fluid can pass from one to the other. 

 Hence it is called the cerebro-spinal fluid. If the blood 

 supply in the head is from any cause excessive, some of this 

 fluid finds its way out of the brain along the cord into the 

 spinal canal. When the blood pressure within the brain is 

 lessened, the fluid wells up again to occupy the space. Thus 

 the equilibrium is constantly maintained. 



10. The Spinal Cord. Safely enclosed in the bony 

 canal formed by the vertebrae is the second portion of the 

 cerebro-spinal centre. It is a cylindrical cord of nerve tissue 

 about three-quarters of an inch in diameter and seventeen 

 inches long, tapering at its lower extremity. It does not 



