634 THE NERVOUS SYSTEM 



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 dif- 

 ference persists 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 1 6 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. 



Cfl 



FIG. 399. 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 per- 

 pendicular fissure very well marked. (Gratiolet.) 



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 i \ oz. ; its weight relatively to the 

 brain is about 1:40 in the adult. As we descend the animal 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. In the newt, 1:105; 

 and in the lamprey, j : 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 

 one-third of 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 convolu- 

 tions in the sides of the fissures. 3. The greater relative size and complexity 

 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 (rostrum) between the olfactory bulbs. 4. The much greater 



