GENERAL ARRANGEMENT OF PARTS OF THE BRAIN 531 



Division of a single anterior pyramid of the medulla at a point just above 

 the decussation is followed by paralysis of voluntary motions in the muscles of 

 the opposite side in all parts below. Disease or division of any part of the 

 cerebro-spinal axis below the seat of decussation of the pyramids is followed 

 by impairment or loss of voluntary motion on the same side of the body. 

 The paralysis is never quite complete, and the opposite side usually shows some 

 slight impairment of function of the muscle. 



When one-half of the spinal cord is cut through in monkeys, the results 

 are as follows (Mott) : Motor paralysis of the muscles of the same side (never 

 complete paralysis of the muscles used in bilateral associated action), followed 

 by gradual recovery of muscular movement, except of the finer movements 

 of the hand and foot; wasting and flabbiness of the muscles; sensory paralysis 

 of the same side (temperature, touch, pain, and pressure) ; temporary vaso- 

 motor paralysis on the same side. The temperature of the affected side is 

 depressed i to 3 F. 



III. THE BRAIN STEM. 



General Arrangement of Parts of the Brain. The great relative 

 and absolute size of the cerebral hemispheres in the adult man and in mammals 

 to a great extent masks the real arrangement of the several parts of the brain. 

 An examination of the accompanying diagram, figures 370, 371, reveals that 

 the parts of the brain are disposed in a linear series, as follows (from before 

 backward): Olfactory lobes, cerebral hemispheres, thalamencephalon (optic 

 thalami and third ventricle), the mid-brain (corpora quadrigemina and crura 

 cerebri), medulla oblongata and cerebellum. 



This linear arrangement of parts actually occurs in an early stage of the 

 development of the human fetus, and it is permanent in some of the lower 

 Vertebrata. In fishes the cerebral hemispheres are represented by a pair 

 of ganglia intervening between the olfactory and the optic lobes, and con- 

 siderably smaller than the latter, their adult development is fairly well repre- 

 sented by the figure 387. In Amphibia the cerebral lobes are further devel- 

 oped, and are larger than any of the other ganglia. 



In reptiles and birds the cerebral ganglia attain a still further development, 

 and in Mammalia the cerebral hemispheres exceed in weight all the rest of the 

 brain. As we ascend the scale, the relative size of the cerebrum increases, till 

 in the higher apes and man the hemispheres, which commenced as two little 

 lateral buds from the anterior cerebral vesicle, have grown upward and back- 

 ward, completely covering in and hiding from view practically all the rest of the 

 brain. At the same time the smooth surface of the cerebral cortex of many 

 lower mammalia, such as the rabbit, is replaced by the labyrinth of convo- 

 lutions of the human brain. 



When the cerebral hemispheres are removed, several large basal masses of 



