863 



they were inimical or indifferent or beneficial to its individual ends. The sensor 

 periphery, in consequence of the demands of evolution, underwent specialization 

 in the development of olfactory and gustatory organs for testing the quality of the 

 food and of the surrounding medium; optic organs for perceiving rays of light; 

 auditory organs for the appreciation of certain oscillations of the surrounding 

 medium; while others, strictly tactile in nature, underwent elaboration as such in 

 the development of sensitive antennae or tentacles. Motor contrivances, useful 

 in the quest for food or in encounters with the enemy, were developed in the way 

 of powerful jaws and masticatory muscles. In brief, a remarkable specialization 

 and differentiation of structure 

 attended the development of the 

 head end, and with it the central 

 organ of control kept pace. In 

 the human species we find certain 

 of these structural characteristics 

 in a highly developed condition, 

 while others have dwindled or 

 disappeared in the course of evo- 

 lution. Thus, in the myxinoid 

 fishes and the lamprey the cere- 

 bral hemispheres themselves are 

 mere appendages of the olfactory 

 lobes; the sense of smell was prob- 

 ably the most important in lower 

 animals. In the brain of man 

 conditions are reversed and the 



Middle peduncle 

 Inferior peduncle 



Medulla oblongata 



FIG. 631. Scheme showing the connection of the severa! 

 parts of the brain. 



olfactory system is seen to have 

 dwindled to an extreme degree as 

 compared with the immense size 

 of the cerebrum; this in conformity with the relatively slight use made of the smell 

 sense in the mental life of man. Other organs of special sense, however, became 

 augmented, and these, together with the nerve mechanisms controlling the vital 

 functions (respiration, circulation), required a more and more elaborate central 

 nerve organ for the harmonious interaction of the several elements. This central 

 organ or brain developed, in bulk and complexity, hand in hand with the increase 

 of the intellectual faculties. Alan's most manifest distinction from other animals 

 has resulted from a remarkable evolutionary growth in brain size and brain 

 power; and as the brain is the material organ of mental and moral manifesta- 

 tions, we find in mankind the highest degree of superiority and culture not 

 only as compared with the nearest related apes, but of the civilized and progressive 

 races as compared with the primitive and unprogressive races. 



DESCRIPTIVE ANATOMY OF THE ADULT HUMAN BRAIN. 



Morphologically considered, the brain consists of a common trunk (or brain 

 axis) from which the two cerebral hemispheres crop out like swollen terminal buds, 

 while the cerebellum is an excrescence of the trunk itself. The axially situated 

 brain axis or "brain stem" 1 comprises, roughly speaking, the axial parts of all 



1 Also "brain isthmus," a loosely used term. It may here be remarked that most extant accounts of the 

 anatomy of the brain overemphasize the distinction of brain parts from each other. Some authors follow 

 one or another system based upon the theories of the segmentation of the brain tube; others divide the brain 

 into (a) rhombencephalon or hind-brain and (/>) cerebrum, comprising mid- and fore-brain. None of the 

 classifications proposed are, as already pointed out, quite satisfactory. The continuity of the parts can only 

 be interrupted arbitrarily, and such procedure leads to a too narrow conception of brain structures single and 

 apart rather than serial and connected. 



