494 MAMMALIA. 



conditions intermediate between these, to wliich such terms as 

 semiplantigrade. semidigitigrade have been applied. 



In the tarsus the tibiale and intermedium are always fused 

 (according to the ordinary view), the centrale is always present, 

 and tarsalia 4 and 5 are fused as are the corresponding bones in 

 the wrist. Tibial and fibular sesamoids are very generally pre- 

 sent, but the tibial sesamoid is not so important as the pisiform 

 of the wrist. But, as in the case of the manus, sesamoid bones 

 may be developed in tendons in other situations, as in the 

 tendons on the plantar surface of the tarsus, on the plantar 

 surface of the metatarso-phalangeal articulation. 



The following table shows the names of the tarsal bones. 



astragalus (talus). 



calcaneum (os calcis). 

 navicular (scaphoideum). 

 internal cuneiform (entocuneiform). 

 middle cuneiform (mesocuneiform) 

 external cuneiform (ectocuneiform). 



cuboid. 



The ankle joint is always between thecrus (tibia and jGLbula)and 

 the tarsus, never between the two rows of tarsal bones as in birds 

 and some reptiles ; and the calcaneum always possesses a well- 

 marked heel process. 



The nervous system is characterised by the size and high de- 

 velopment of the cerebrum, the hemispheres of which are so large 

 that they not only fill the anterior part of the cranial cavity but 

 even partly cover the cerebellum (Fig. 263). In Ornithorhyn- 

 chus, various small rodents and insectivores and some of the 

 smaller primates the surface of the hemispheres is smooth or 

 nearly so (Fig. 263, a), but in most Mammalia it is marked by 

 depressions and ridges, which in the higher forms (Fig. 263, d) 

 become furrows or fissures {sulci) and convolutions (gyri). The 

 number and complexity of the convolutions may be said, sjDeak- 

 ing generally, to vary directly with the intelligence of the animal, 

 but they seem, in some cases at least, to depend upon the size of 

 the animal, for we frequently find that in the smaller members 

 of a group the convolutions are less marked than in the larger. 



