THE BRAIN. 483 



distal portion of the nerve is stimulated, no effect is produced, 

 while stimulation of the proximal portion produces sensation. 

 The posterior roots are therefore afferent and sensory and are dis- 

 tributed to the skin. The two roots uniting form a mixed nerve 

 that is, one in which there are both motor and sensory fibers. 



Recurrent Sensibility. When the distal end of a divided an- 

 terior root is stimulated, besides the muscular contraction which 

 follows there is also some pain produced. If the trunk of the 

 nerve beyond the ganglion is divided, and then the anterior root 

 is stimulated, no muscular contraction results, but the pain is felt 

 as before. If, however, the posterior root is divided, no sensation 

 is produced. The sensation experienced when the anterior root is 

 stimulated is accounted for by the presence in this root of some 

 sensory fibers which pass up into it for a short distance and form 

 a loop, returning to the junction of the two roots, and then pur- 

 suing their course in the posterior root. These are called recurrent 

 sensory fibers. The impulse passes along these fibers to the point 

 of junction of the two roots, and then along the posterior root to 

 the nerve-center. 



Function of the Spinal Ganglia. As has already been stated, 

 upon each posterior root of a spinal nerve, with one exception, is 

 a ganglion. When examined under the microscope, the root-fibers 

 spread out, passing between groups of large cells having promi- 

 nent nuclei and a diameter of about 100 JJL. With one of these 

 ganglion-cells a root-fiber is in communication, and the function 

 of these cells is to form the fibers and to regulate their nutrition ; 

 they are true trophic centers. 



THE BRAIN. 



The brain y or encephalon (Figs. 278, 279), is that part of the 

 cerebrospinal axis situated within the cranium or skull. Its 

 divisions are sometimes described as the forebrain, including the 

 hemispheres, with the olfactory lobe, the corpora striata, and the 

 optic thalami ; the midbrain, being the corpora quadrigemina and 

 the crura cerebri ; and the hindbrain that is, the cerebellum, the 

 pons Yarolii, and the medulla oblongata. 



In the adult male the brain weighs, on an average, 1415 grams ; 

 its weight in the female is about 1245 grams. In 278 cases of males 

 in which the brain was weighed the maximum was 1841 grams 

 and the minimum 963 grams. In 191 cases of females the max- 

 imum was 1586 grams and the minimum 878 grams. The brain 

 of Cuvier, the great naturalist, weighed 1815 grams; that of an 

 idiot weighed 651 grams. The brain of a mulatto not remarkable 

 for intelligence weighed 1927 grams. The forebrain weighs about 

 1245 grams in the adult male. 



The gray matter of the brain is in some parts on the surface, 



