574 



THE NERVOUS SYSTEM 



Weight of the Brain and Cord. The brain of an adult man weighs from 48 to 50 

 oz. about 1,550 grams, or about 2 per cent of the body weight. It exceeds in absolute 

 weight that of all the lower animals except the elephant and whale. Its weight, relatively 

 to that of the body, is exceeded only by that of a few small birds, and some of the smaller 

 monkeys. 



In the new-born child the brain (weighing 10 to 14 oz.) is about 10 per cent of the 

 weight. At the age of 7 years the weight of the brain already averages 40 oz., and about 

 14 years the brain not infrequently reaches the weight of 48 oz. Beyond the age of forty 

 years the weight slowly but steadily declines at the rate of about i oz. in 10 years. 



The average weight of the female brain is less than the male; and this difference per- 

 sists from birth throughout life. The difference amounts to about 5 oz. Thus the average 

 weight of an adult woman's brain is about 44 oz. 



The brains of idiots are generally much below the average, some weighing less than 16 

 oz. Still the facts at present collected do not warrant more than a very general statement, 



FIG. 406. Brain of the Orang, $ Natural Size, Showing the Arrangement of the Convolutions. 

 Sy, Fissure of Sylvius; R, fissure of Rolando; EP, external perpendicular fissure; Olf, olfactory 

 lobe; Cb, cerebellum; PV, pons Varolii; MO, medulla oblongata. As contrasted with the 

 human brain, the frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the 

 temporo-sphenoidal lobe very prominent, and the external perpendicular fissure very well marked. 

 (Gratiolet.) 



to which there are numerous exceptions, that the brain weight corresponds to some extent 

 with the degree of intelligence. There can be little doubt that the complexity and depth 

 of the convolutions, which indicate the area of the gray matter of the cortex, correspond 

 with the degree of intelligence. 



The spinal cord of man weighs from i to ij oz.; its weight relatively to the brain is 

 about i : 40 in the adult. As we descend the animal scale, this ratio constantly increases 

 till in the mouse it is i : 4. In cold-blooded animals the relation is reversed, the spinal 

 cord is the heavier. In the newt, 1 105; and in the lamprey, i : 133. 



The most distinctive points in the human brain, as contrasted with that of apes, are: 

 i. The much greater size and weight of the whole brain. The brain of a full-grown 

 gorilla weighs only about 15 oz. (450 grms.), which is less than ^ the weight of the human 

 adult male brain, and barely exceeds that of the human infant at birth. 2. The much 

 greater complexity of the convolutions, especially the existence in the human brain of 

 tertiary convolutions in the sides of the fissures. 3. The greater relative size and complex- 

 ity and the blunted quadrangular contour of the frontal lobes in man, which are relatively 

 broader, longer, and higher than in apes. In apes the frontal lobes project keel-like 



