590 Dynamic Theory, 



ing in the case of the elephant, the right and left descending veins are 

 united into one before discharging into the right auricle. The gyren- 

 cephalic brain is usually accompanied by a better plrysical development, 

 either greater size, or strength, or activity. The smaller members of 

 each family are apt to possess less highly developed cerebrums than 

 those of the larger members. The difference in the activities of the 

 mammal cerebrum is indicated by the convolutions and fissures. By 

 the help of the figures of convolutions, for which we are chiefly in- 

 debted to Owen, comparison can be easily made. The first fissure de- 

 veloped on the outer surface of the cerebrum is the great longitudinal 

 fissure which separates the two hemispheres. This fissure is found in all 

 the vertebrates except some of the fishes. It is not equally complete in all. 



FIG. 306. Inner surface of hemisphere of 

 Brain of Ornithorhynchus. 

 #. Olfactory lobe. 

 (76. Cerebellum, 

 a. Anterior plliars of the fornix. 

 o. Comn.iobUi'c of the pillars of the Fornix 

 p. Posterior pillars of the Fornix. [(Lyra). 

 4. Hippocampal fissure, 

 c. Anterior commissure of the brain. 



FIG. 306. 

 In the Ornithorhynchus there is formed for the first time the hippo- 



campal fissure, No. 4, which is formed by the pushing in of the hippo- 

 campus from the inner, or mesial, surface of the hemisphere. Aside 

 from this, there is little or no break in the smoothness of the cerebrum. 

 The brain of the echidna is considerably convoluted. Of the regular 

 convolutions found in the higher families he has beside No. 4, the super 

 callosal No. 7 ' . Most of the others remain unidentified with those of 



