XIII 



PHYLUM CHORDATA 



547 



In the former the cerebral hemispheres are relatively small, do not 

 overlap the cerebellum, and have smooth, or nearly smooth, sur- 

 face. In the latter the relative 

 development of the hemispheres 

 is immense, and their backward 



FIG. 1136. Brain of Ornithorhynchus FIG. 1137. Brain of Echidna aculeata, 



anatinus , dorsal view (natural size) ; cbl. dorsal view (natural size), 



cerebellum ; olf. olfactory lobes. 



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

 while the cortex is thrown into numerous complicated convolu- 

 tions separated by deep sulci 

 (Fig. 1139). This development 

 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. Jacobson's 

 organs, which in the Sauropsida 

 constitute such important acces- 

 sory parts to the olfactory ap- 

 paratus, are well developed only 

 in the lower groups of Mam- 

 mals. 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 



. , -, FIG. 1138. Brain of Kangaroo (Macropus 



among Mammals do the nasal major). (After Owen.) 



chambers lose their sensory 



functions the olfactory nerves being vestigial or absent. The 

 organs of taste are taste-bulbs in the mucous membrane covering 

 certain of the papillae on the surface of the tongue. 



N N 2 



