562 



ZOOLOGY 



SECT. 



FiG.ji2i9. Brain of Kangaroo 



major). (After Oiven.) 



in the Eutheria by the fibres of the corpus callosum, and the ventral 

 part persists in the shape of the psalterium or lyra. 



In Ornithorhyuchus (Fig. 1217) the hemispheres are smooth ; in 

 Echidna (Fig. 1218) they are tolerably richly convoluted. Both 



genera, but more particularly 

 Echidna, are characterised by the 

 enormous development of the 

 parts of the hemispheres (archi- 

 palliurn) connected with the 

 olfactory sense. In the lower Mar- 

 supials there are no convolutions 

 (Notoryctes, Koala, Phalangers), 

 while in the higher the convolu- 

 tions are numerous, though the 

 sulci are not very deep (Macropus, 

 Fig. 1219). Among the Eutheria 

 there is a great range in the grade 

 of development of the brain, 

 from the Rodents and lower In- 

 sectivores to the higher Primates. 

 In the lower types of Mammalian 

 brain the cerebral hemispheres 

 are relatively small, do not over- 

 lap the cerebellum, and have 

 smooth, or nearly smooth, surfaces. In the higher types the 

 relative development of the hemispheres is immense, and their 

 backward extension causes them to cover over all the rest of the 

 brain, while the cortex is thrown into numerous complicated 

 convolutions separated 

 by deep sulci (Fig. 

 1220). This develop- 

 ment of the cerebral 

 hemispheres reaches its 

 maximum in Man. 



The organs of 

 special sense have the 

 same general structure 

 and arrangement as in 

 the Sauropsida. Jacob- 

 son's organs, which in 

 the Sauropsida consti- 

 tute such important 

 accessory parts to the 

 olfactory apparatus, are well developed only in the lower groups 

 of Mammals. The olfactory mucous membrane is of great extent, 

 owing to the development of the convoluted ethmo-turbinal bones 

 over which it extends. In the toothed Cetacea alone among 



FIG. 1220. Dorsal view of brain of Gray's Whale 

 (Cogia grayi). (After Haswell.) 



