COURSE OF THE MOTOR AND SENSORY TRACTS. 741 



Unilateral or total division of the spinal cord at progressively higher levels 

 paralyzes, accordingly, respiratory nerves arising at successively higher levels on 

 the same side or on both sides. 



Pathological. In case of degeneration or direct injury of the spinal cord or 

 of individual portions of the cord, it should be especially noted that occasionally 

 in recent cases irritative and paralytic phenomena occur side by side in closely 

 adjacent portions of the cord, and as a result an analysis of the clinical picture 

 is rendered difficult. 



Degeneration of the posterior columns, without involvement of the afferent 

 posterior roots, causes loss of tactile sensibility as the most conspicuous symptom, 

 while thermal sensibility is preserved. Degeneration of the ganglion-cells in the 

 anterior horns, for example in cases of spinal paralysis of infants, gives rise to 

 paralysis in the distribution of the efferent motor nerves. At the same time the 

 muscles supplied by these nerves undergo rapid atrophy. The ganglion-cells are 

 the trophic centers for the nerves and the muscles. The results, under such cir- 

 cumstances, are the same as those that follow permanent division of a peripheral 

 motor nerve. As some fibers pass from above downward through the anterior 

 horn to the opposite side, also some fibers on the contralateral side degenerate. 

 Degeneration of the posterior gray horns gives rise to impairment of cutaneous 

 sensibility and to trophic disorders in the skin. Degeneration of the central por- 

 tion of the gray matter causes, in addition to trophic disorders in the skin, loss 

 of thermal sensibility. 



It is a highly interesting fact that temporary occlusion of the abdominal aorta, 

 in rabbits, causes permanent sensory and motor paralysis in the entire area con- 

 trolled by the portion of the spinal cord whose circulation is cut off. Ganglion- 

 cells and nerve-fibers of the anterior horns undergo degeneration. Then secondary 

 degeneration of the anterior roots follows (with the exception of the contained 

 vasomotor fibers), and of the white matter adjacent to the anterior horns. Sub- 

 sequently the posterior horns also undergo reduction in size. All of the tracts 

 passing into the cord remain intact, namely the posterior roots, the spinal ganglion- 

 cells, the posterior columns and the extreme periphery of the anterolateral tract. 



Destruction of the lower portion of the spinal cord, in the dog, up to the 

 cervical cord gives rise, in addition to loss of sensation and of motion, to reduction 

 in the temperature of the lower portion of the body, but, if great care be taken, 

 to no trophic disorder, except that the bones appear to be brittle. The anus is 

 dilated only at first, but later the sphincter regains its normal tone and sponta- 

 neously contracts rhythmically (perhaps it contains its innervational center within 

 itself), while all the other paralyzed muscles of the body undergo degeneration. 

 The digestive processes, the intestinal movements, the act of parturition, the act 

 of nursing are performed normally; but the temperature of the body can be 

 regulated only within certain limits. The paralysis of the bladder present at 

 first improves; the tone of the blood-vessels is restored in the course of a few 

 days. A series of important vital processes are, therefore, not directly dependent 

 upon the existence of the spinal cord, but they are rather decentralized. 



THE BRAIN. 



GENERAL OUTLINE OF THE STRUCTURE OF THE BRAIN. 



COURSE OF THE MOTOR AND SENSORY TRACTS. 



With respect to an organ exhibiting such a high degree of complexity of 

 structure as the brain, it is of the greatest importance to be acquainted with its 

 general arrangement, even if only from a brief description. It is to the credit 

 of Meynert that he devised a practical system of this character based on extensive 

 investigations. This will be used in the discussion of the subject that follows, 

 although consideration will be given also to the results of more recent investi- 

 gation. , 



The weight of the brain in man is on the average in the male 1372 grams, 

 in the female 1231 grams. The uppermost and outermost layer of the cortex 

 consists of a layer of glia containing nerve-fibers. Beneath this is the layer of 

 small ganglion-cells, and next to this the layer of large pyramidal cells, which 



