152 PRINCIPLES OF ANIMAL BIOLOGY 



lesions due to accident or disease in man, and, recently, the study of 

 "action potentials," which mark the path of nerve impulses from the 

 point of stimulation to their center. The latter method is particularly 

 useful in locating functional areas in the cerebrum. To understand 

 what follows, it is necessary to know the general structure of the brain. 



As it originates in the embryo, the central nervous system is a tube, 

 wider in front where the brain develops (page 208), narrower behind 

 in the spinal cord. The brain tube enlarges moderately in three regions 

 known as the fore-, mid-, and hindbrain. This tubular structure remains 

 in the adult as the "brain stem," but the forebrain expands enormously 

 upward, laterally, and backward, to form the cerebrum (divided into two 

 hemispheres), while the hindbrain develops the cerebellum. Behind the 

 latter is the medulla oblongata, which is usually counted a part of the 

 brain but is really the somewhat enlarged anterior end of the spinal cord. 



The cerebrum has a gray surface layer, the cortex — gray because 

 of the cell bodies which it contains — which in man and the mammals 

 generally is greatly increased in extent by folds and furrows. It is 

 the cortex which has been the subject of much of the localization study, 

 because it is the seat of those psychic qualities which tend to distinguish 

 man from the beasts. By the methods outlined above, the functions of 

 various parts of the cerebral cortex have been found to be roughly as 

 portrayed in Fig. 125. The best established of the areas there shown 

 are the motor area and the area of skin sensation which together form a 

 transverse band halfway between the front and rear, the areas for hearing 

 at the sides, and that for vision at the extreme posterior part. The rest 

 of the cerebrum is largely given over to what may be termed associations, 

 some of the particular forms of which are indicated in the illustration. 

 The association areas deal with integration of individual sensations into 

 a whole. The cortex is not responsible for pain except to localize it, 

 and it is not concerned with any viscer^il sensations such as hunger and 

 thirst. Pain is a function of the thalamus, in the stem region of the 

 forel)rain. 



The cerebellum serves to coordinate muscular actions. Destruction 

 of it results in irregular, jerky, fumbling, or reeling movement, or in 

 thick slurred speech. The middle portion influences muscles of the 

 trunk, neck, and head; each side of the cerebellum acts on muscles of the 

 same side of the body, but there is not much other known localization. 



The more important functions of the medulla in controlling the heart 

 and digestive canal, the contraction and dilation of blood vessels, and the 

 movements in breathing have already been described in this and earlier 

 chapters. 



Chemical Regulation. — The control of vital actions by the medulla 

 is exercised partly at the behest of accumulated carbon dioxide. It has 



