126 



HAND-BOOK OF PHYSIOLOGY. 



In Reptiles and Birds the cerebral ganglia attain a still further de- 

 velopment, and in Mammalia the cerebral hemispheres exceed in weight 

 all the rest of the brain. As we ascend the scale, the relative size of the 

 cerebrum increases, till in the higher apes and man the hemispheres, 

 which commenced as two little lateral buds from the anterior cerebral 

 vesicle, have grown upward and backward, completely covering in and 

 hiding from view all the rest of the brain. At the same time the smooth 

 surface of the brain, in many lower Mammalia, such as the rabbit, is re- 

 placed by the labyrinth of convolutions of the human brain. 



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

 50 oz. or about 3 Ibs. 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 only exceeded by that of a few small birds and some 

 of the smaller monkeys. In the adult man it ranges from ^L- -fa of the 

 body weight. 



Variations. Age. In a new-born child the brain (weighing 10 14 oz.) 

 is iV of the body weight. At the age of 7 years the weight of the brain 



FIG. 339. Longitudinal and vertical Diagrammatic section of a Vertebrate brain. Letters as be- 

 fore. Lamina terminalis is represented by the strong black line joining Pn and Py. (Huxley.) 



already averages 40 oz., and about 14 years the brain not unfrequently 

 reaches the weight of 48 oz. Beyond the age of 40 years the weight 

 slowly but steadily declines at the rate of about 1 oz. in 10 years. 



Sex. The average weight of the female brain is less than the male; 

 and this difference persists from birth throughout life. In the adult it 

 amounts to about 5 oz. Thus the average weight of an adult woman's 

 brain is about 44 oz. 



Intelligence. The brains of idiots are generally much below the aver- 

 age, some weighing less than 16 oz. Still the facts at present collected 

 do not warrant more than a very general statement, 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 com- 

 plexity and depth of the convolutions, which indicate the area of the grey 

 matter of the cortex, correspond with the degree of intelligence (E. 

 Wagner). 



Weight of the Spinal Cord. The spinal cord of man weighs from 

 11 ^ oz. ; its weight relatively to the brain is about 1 : 36. As we descend 

 the scale, this ratio constantly increases till in the mouse it is 1 : 4. In 

 cold-blooded animals the relation is reversed, the spinal cord is the heavier 

 and more important organ. In the newt, 2:1; and in the lamprey, 75 : 1. 



Distinctive Characters of the Human Brain. The following character 

 distinguish the brain of man and apes from those of all other animals, (a.) 



