480 Vertebrata. 



A general cliaracteristic of the mammalian skeleton consists in the oecuiTenoe 

 of special ossifications for the end portions of many bones, especially of long 

 bones, and also of many processes, so that in young animals many bones consist 

 of several pieces, which fuse later. These special tenninal ossifications are 

 termed epiphyses. 



The brain is cliaracteristic in many respects. The cerebrum 

 is of considerable size; its surface is marked by deep labyrinthine 

 furrows, the sulci, separated from each other by ridges, the gyri : 

 ■occasionally the surface is smooth, or almost smooth, as in the 

 Kodents. The hemispheres cover not only the thalamencephalon, 

 but usually the mid-brain also, and sometimes part of the cerebellum. 

 Peculiar to the Mammalia is the so-called corpus callosum, an 

 important system of transverse nerve fibres, which pass from one 

 hemisphere to the other; these fibres are least developed in the 

 Monotremes and Marsupials. The mid-brain is divided not only 

 by a longitudinal, but also by a transverse furrow, so that it forms 

 iour dorsal ]obes {corpora quadrigemina) . The cerebellum is well 

 developed, its much thickened dorsal wall is divided into two median 

 and two lateral portions, and is transversely folded. 



The size of the brain, as compared with that of the rest of the body, is closely 

 ■correlated with the intellectual level of the species under consideration (see, for 

 instance, the enormous development in Man). There are, however, other circiom- 

 stances which are of great importance in this connection : noticeably it is a rule 

 that small Mammals have relatively larger brains than have their nearest 

 allies ; it may be said that, on the whole, the size of the brain varies inversely as 

 the bulk of the animal; the Elephant, for instance, in spite of its stiildng 

 intellectual qualities, has a relatively minute brain. It may also be noticed here 

 that the brain of young animals is relatively larger than that of the adult. 



Olfactory organs. Prominent folds, the turbinal.s, 

 project into the nasal cavities, which are usually of considerable size. 

 The turbinals arise from the postero-external wall, developing as 

 large lamellse, which become folded and coiled so as to form very 

 complex structures. They are supported at first by cartilage, which 

 becomes partly or entirely ossified, to form the maxillo- and the ethmo- 

 iurbinals. The olfactory epithelium is situated in that part 

 of the mucous membrane which lines tlae region of the nasal cavity 

 nearest tlie cribriform plate; it is yellowish-brown in colour. The rest 

 of the cavity has no olfactory significance ; in additiou to mucu-;- 

 secreting glands it has a rich vascular network which, according to 

 some authorities, serves to warm the air entering the lungs. The air 

 sacs of some of the skull bones mentioned above (p. 475-6) are out- 

 growths from the nasal cavity, and are lined with a continuation of 

 its mucous membrane. 



Optic Organs. In contrast to other Yerteb rata the upper 

 eyelid is larger and more movable than the lower. A nictitating 

 membrane is always present but less well developed than in Birds 

 .and Eeptiles, and usually not provided with special muscles ; it slips 



