SIZE OF BRAIN IN MAMMALIA. 143 



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

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

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

 the fissure called e 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 musculus)', 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 Ib. avoird. ; that of a Whale (Bal&nopterci) 100 

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

 brain of a pygmy Chevrotain ( Tragulus pygm&us) 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 Ibs. 12 oz. 



8 xcvii-. 4 v". 



