SIZE OF BRAIN IN MAMMALIA. 143 



enters the ( ventricle ' becomes contracted to a very small extent 

 of the base exterior to the crus. From this point begins the fold 

 extending, as ( choroid plexus/ from one ventricle to the other by 

 the fissure called ' foramen Monroianum ' in Anthropotomy. On 

 the interior surface of the hemisphere the pia mater is reduced to 

 an epithelium, the cells of which are less flat in the lateral ven- 

 tricles than in that continuation therefrom called ( third ventricle.' 

 The part of the interhemispheral fissure overarched by the great 

 transverse commissure is the ' fifth ventricle.' For other dif- 

 ferentiated and definite parts in the archencephalous brain — 

 the subjects of the 'bizarre' nomenclature of Anthropotomy — 

 reference may be made to the minute and exact monographs 

 which have been published on that part of the human structure. 



§ 209. Size of Brain. — The brain grows more rapidly than the 

 body, and is larger in proportion thereto at birth than at full 

 growth. But there is a difference in this respect in different 

 Mammalian orders. The brain of the new-born Marsupial is less 

 developed relatively than in higher Mammals, and grows more 

 gradually or equally with the subsequent growth of the body. 1 

 So, in the degree in which a species retains the immature character 

 of dwarfishness, the brain is relatively larger to the body : it is as 

 1 to 25 in the pygmy Petaurist, but is as 1 to 800 in the Great 

 Kangaroo ; it is as 1 to 20 in the Harvest Mouse, but is as 1 to 

 300 in the Capybara ; it is as 1 to 60 in the little two-toed 

 Ant-eater, and is as 1 to 500 in the Great Ant-eater. The 

 brain weighs 6 grains in the Harvest Mouse (Mus messorius), 

 and the same in the Common Mouse (Mus muscalus) ; but the 

 weight of the Harvest Mouse is 112 grains, whilst that of the 

 Common Mouse is 327 grains. The brain of a Porpoise, 4 feet 

 long, may weigh 1 lb. avoird. ; that of a Whale (Balcenoptera) 100 

 feet in length does not exceed 4 lbs. avoird. 2 In Artiodactyles the 

 brain of a pygmy Chevrotain ( Tragulus pygmceus) is to the body 

 as 1 to 80; in the Giraffe 3 it is as 1 to 800. In Perissodactyles 

 the brain of the Hyrax is as 1 to 95, whilst that of the Indian 

 Rhinoceros is as 1 to 764. 4 The brain of the Elephant may be 

 three times heavier than that of the Rhinoceros, but a full-grown 

 male would probably weigh down four Rhinoceroses. In Car- 

 nivora the brain of the Weasel is to the body as 1 to 90 ; in the 

 Grisly Bear it is as 1 to 500 ; in Quadrumana the brain of the 



1 lxxv', p. 347, pi. vii. figs. 9-12. 



2 Scoresby, in a Balcena mysticetus of 65 feet in length, found the weight of the 

 brain to be 3 lbs. 12 oz. 



8 xcvii-. 4 x". 



