Chap. XX.] OF THE HUMAN BRAIN. 355 



Thurnam from a careful consideration of previously 

 recorded results comes to the following conclusions : — 



" It may in general be admitted that the average weight of the 

 brain undergoes a progressive increase to a period somewhere 

 between the twentieth and fortieth year. According to all the 

 tables before us which refer to the sane, the greatest average 

 weight for the male brain is that for the middle decennial period, 

 or from thirty to forty years ; and this, as M. Broca observes, 

 agrees perfectly with what we know of the continued develop- 

 ment of intelligence during the whole of this period. For wotneu 

 the full average size of the brain is perhaps attained within the 

 preceding decade of twenty to thirty years ; but the difference 

 between the two sexes in this respect is not great. From forty 

 to fifty years there is a slight diminution in weight and a 

 greater one between fifty and sixty. After sixty years the rate 

 of decrease is still greater; the process of decay becomes more 

 and more rapid, and thus in the eighth decade of existence the 

 average weight of the brain is less by more than three ounces (80 

 to 90 grammes) than it was in the fourth decade. In the aged, on 

 the average, the weight of the brain decreases '[lari passu with the 

 intelligence. There are many exceptions to this general law, and 

 some, particularly of the more cultivated and learned class, pre- 

 serve to extreme age all the fulness and vigour of their faculties. 

 The brain of such men, as the late Professor Gratiolet observes, 

 remains in a state of perpetual youth, and loses little or none of 

 the weight which belonged to it in the prime of life." 



Sex. — Thurnam says : — " My own observations fully 

 confirm those of preceding writers as to the average 

 weight of the adult male brain being about ten per cent, 

 greater than that of the female. As Professor Welcker 

 expresses it : ' The brain- weight of the male (1390 grmm.) 

 is to that of the female (1250 grmm.)* as 100 : 90.' 

 Slight variations are observable in the brain-weights of 

 the two sexes, as given by different observers, but it will 

 be seen that the average difference is expressed with 

 much accuracy by these figures." 



* That is about 49 oz. and 44 oz. respectively. 



