120 Comparative View of 



The optic nen'e in the mole is a slender thread, and its vision 

 is feeble ; the same nerve is large and thick in the eagle, ac- 

 companied by intense powers of sight. Again, the fact admits 

 of demonstration, that deficiency in the size of the brain is 

 one, although not the only, cause of idiocy. Although the 

 brain be healthy, if the horizontal circumference of the head, 

 with the muscular integuments, do not exceed thirteen or 

 fourteen inches, idiocy is the invariable consequence. Dr 

 Voisin states that he made observations on the idiots under his 

 care at the Parisian Hospital of Incurables, and found that in 

 the lowest class of idiots, where the intellectual manifestations 

 were null, the horizontal circumference, taken a little higher 

 than the orbit, varied from eleven to thirteen inches, while 

 the distance from the root of the nose backwards, over the 

 top of the head, to the occipital spine, was only between 

 eight and nine inches ; and he found no exception to this fact. 

 If, therefore, extreme defect of size in the brain be invariably 

 accompanied by mental imbecility, it is a legitimate inference 

 that size will influence the power of manifestation through all 

 other gradations of magnitude, always assuming other condi- 

 tions to be equal. 



Physiological authorities are equally explicit on this subject. 

 Magendie says, " the volume of the brain is generally in direct 

 proportion to the capacity of the mind. We ought not to sup- 

 pose, however, that every man having a large head is neces- 

 sarily a person of superior intelligence ; for there are many 

 causes of an augmentation of the volume of the head besides 

 the size of the brain ; but it is rarely found that a man dis- 

 tinguished by his mental faculties has not a large head. The 

 only way of estimating the volume of the brain, in a living 

 person, is to measure the dimensions of the skull ; every other 

 means, even that proposed by Camper, is uncertain." 



The difference of mental power between young and adult 

 minds, is a matter of common observation. The difference in 

 the weights of their brains is equally decided. 



According to Cruveilhier, in three young subjects, the 

 weights of the brains were as follows : 



In the first, the brain weighed 2 lb. 2 oz. ; the cerebellum, 

 4i oz. ; together, 2 lb. 6J oz. In the second, the brain weighed 



