624 HANDBOOK OF PHYSIOLOGY. 



bral hemispheres, optic thalami, and third ventricle, corpora quadri- 

 gemina, or optic lobes, cerebellum, medulla oblongata. 



This linear arrangement of parts actually occurs in the human fretus; 

 and it is permanent in some of the lower Vertebrata, e.g., Fishes, in 

 which the cerebral hemispheres are represented by a pair of ganglia 

 intervening between the olfactory and the optic lobes, and considerably 

 smaller than the latter. In Amphibia the cerebral lobes are furthei 

 developed, and are larger than any of the other ganglia. 



In reptiles and birds the cerebral ganglia attain a still further devel- 

 opment, 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 



Fig. 376. Longitudinal and vertical diagrammatic section of a vertebrate brain. Letters 

 as before. Lamina terminalis is represented by the strong black line joining Pn and Py. 

 (Huxley.) 



surface of the brain, in many lower mammalia, such as the rabbit, is 

 replaced 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. (about 1550 grms.) . 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 3^ -fa of the body weight. 



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

 fa of the body 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 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 average, 

 some weighing less than 16 oz. Still the facts at present collected do not war- 

 rant more than a very general statement, to which there are numerous excep- 

 tions, 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 



