Sauropsida: Class Ileptilia 479 



of aquatic anamniotes. This is in accord with the fact that the reptile 

 has the more highly differentiated and complex muscular system and, 

 in general, is capable of greater range and diversity of activities. A 

 greater number of nervous elements and their arrangement in a more 

 elaborate pattern are therefore necessary. The most noteworthy point 

 concerns the most anterior (telencephalon) of the five regions (Fig. 

 370). It consists mainly of a pair of large lobes, the cerebral hemi- 

 spheres. The roof, pallium, of each hemisphere is thinner than the 

 lateral and ventral walls, which together constitute the corpus stria- 

 tum. The pallium, however, is somewhat thicker than the correspond- 

 ing part of the amphibian brain. In fishes the pallium is comparatively 

 thin. In the largest and most specialized group, Teleostei, it is exceed- 

 ingly thin and contains no nervous tissue whatever (Fig. 324). The 

 amphibian pallium however, consists mainly of nervous tissue, most 

 of whose cell-bodies are massed into a deep layer of "gray substance" 

 while an external layer of "white substance" consists mainly of the 

 nerve-fibers. In the reptilian pallium the relative position of "gray" 

 and "white" tends to be the reverse of that in the amphibian (Fig. 

 371). The fibers are mostly deep and the bodies of the nerve-cells are 

 more or less massed into an external layer of "gray" corresponding to 

 the more strongly differentiated external gray layer, the cortex 

 cerebri, of the mammalian hemispheres. 



The cerebral cortex of mammals is the seat of nervous mechanisms 

 which exert the supreme control over the voluntary activities of the 

 body. To a large extent, bodily activities are carried out automatically. 

 We breathe, cough, andsneeze "without thinking." The central nervous 

 mechanisms which immediately control such automatic activities are 

 located in the several parts of the brain other than the cerebral cortex, 

 in the spinal cord, and in the more deeply situated autonomic nervous 

 system. But if a person "stops to think" about it, he may (within 

 limits) hold his breath or suppress the cough or (less likely) the sneeze. 

 Walking and running require that numerous muscles shall alternately 

 contract and relax in perfectly harmonious coordination. Such co- 

 ordination is automatically effected by nerve-centers situated in the 

 cerebellum. But when we may walk and where we go and whether we 

 walk or run are usually determined by something that happens in the 

 cortex cerebri. The general orders having gone forth from the cortex, 

 the routine details of locomotion are managed automatically by the 

 cerebellum. Thinking, conscious deliberation with consequent choice 

 of one line of action rather than another, and activities which we 

 describe as "intelligent," involve nervous activity in the cortex. 



Study of the behavior of fishes, amphibians, and reptiles makes it 

 quite clear that their activities are predominantly the outcome of the 



