BLOOD SUPPLY OF BRAIN 223 



per cent, of water, and the white matter about 70 per cent. 

 The amount diminishes from birth to maturity. Between the 

 fiftieth and sixtieth years the brain loses in weight in all parts, 

 but the loss in the cerebral hemispheres is slightly greater than 

 in other portions. The entire cerebral cortex diminishes in 

 thickness with age, but more in motor than in sensory areas. 

 In the cerebellum the branches of the arbor vitse decrease in 

 number and size; some of the cells are found to be shrunken, 

 while the cells of Purkinje disappear altogether. In the cord, 

 especially in the lumbar region, the cells become shrunken, 

 pigmented, and degenerate; the supporting tissue is increased, 

 and the walls of the bloodvessels are thickened. In paralysis 

 agitans similar but more marked changes occur. With the 

 advance of age the entire central nervous system breaks down 

 into groups of elements, so that its powers are lost in an irregular 

 and disjointed manner. 



Fatigue. When by voluntary contractions of the muscles of 

 the index finger a moderately heavy weight is raised at regular 

 intervals, it is found that the height of the contractions gradually 

 decreases, so that in time the weight can no longer be lifted. 

 If the effort is continued, it is found, in some people at least, 

 that the power of contraction returns periodically to nearly its 

 normal strength, so that a record of the contractions presents a 

 series of waves. The local feeling of fatigue in this case is prob- 

 ably due, to a great extent, to the organs of muscular sense. 

 An explanation of the general fatigue which may result is to be 

 found in Mosso's experiment, in which he injected the blood of 

 a fatigued animal into the circulation of a normal one, giving 

 rise in the latter to all the symptoms of fatigue. It cannot be 

 doubted that muscular activity gives rise to products, which 

 are carried to the brain in the circulation. Lactic acid and the 

 monophosphates of alkali metals circulated through a muscle 

 produce in it all the characters of typical fatigue. Lactic acid, 

 although formed in muscle, is, however, not alone responsible. 



Blood Supply of Brain. In general, the capillary network is 

 closest in the gray matter or wherever any aggregation of cell 

 bodies is to be found. Huber has demonstrated nerve fibers in 

 the walls of the vessels of the pia, and Kolliker claims to have 

 traced them to vessels of the nervous substance proper. Vaso- 

 motor phenomena of the brain have been demonstrated physio- 

 logically. 



