286 INTRODUCTION TO NEUROLOGY 



the postcentral gyrus, to the motor cortical centers of the precentral gyrus, 

 and to other widely separated parts of the cortex. The significance of this 

 fact is still obscure. That the postcentral gyrus is of different functional 

 type from the precentral gyrus is shown by the fact that motor projection 

 fibers arise from the latter and not from the former, by the differences in 

 anatomical structure of these regions, by a large amount of experimental 

 and clinical evidence which shows that tactile sensibility is not lost by the 

 destruction of the precentral motor areas, and finally by direct physiological 

 experiment upon human subjects. 



Dr. Harvey Gushing (1909), in operating upon brain tumors in 2 cases in 

 which the use of an anesthetic was prohibited by the condition of the patient, 

 exposed the postcentral gyrus and, with the patient's consent, electrically 

 stimulated its surface. The patients, who were fully conscious during the 

 operation, reported distinct cutaneous sensations which were subjectively 

 localized as if coming from the skin of the hand. There were no motor 

 responses from this and adjacent parts of the cortex behind the central 

 sulcus, though in the same cases, upon stimulation of the precentral gyrus, 

 motor responses were obtained which were accompanied by no sensations 

 save those which came from the muscles during their contraction. In a 

 previous similar case Dr. Gushing (1908) obtained typical motor responses 

 from stimulation (with the patient's consent) of the precentral gyrus in an 

 operation without anesthesia, and these responses were unaccompanied by 

 painful sensations. 



A very extensive series of experiments involving the stimulation and 

 extirpation of these cortical areas in apes, dogs, and other animals supports 

 the conclusion that the postcentral gyrus is the great receptive center for 

 cutaneous reactions of the general cutaneous system. What may be the 

 functions of those thalamic fibers which pass to the motor centers in front 

 of the central fissure is unsettled. Possibly these connections are concerned 

 in cortical reflexes of the proprioceptive system or acquired automatisms. 



The myelinated fibers of the cerebral hemisphere mature, 

 that is, acquire their myelin sheaths, at various stages in the 

 development of the brain, some of these systems of fibers appear- 

 ing before birth and some after birth. Much investigation has 

 been directed to the determination of the exact facts regarding 

 the sequence of development of these fibers, and many interest- 

 ing theories have been developed regarding the significance of 

 these facts. 



Flechsig in a long series of researches made the first thorough study of 

 this problem, and his conclusions have exerted a profound influence upon 

 all subsequent theories of the functions of the cerebral cortex. He proposed 

 a series of laws of developmental sequence (myelogeny) of the cortical fibers, 

 among which two may be mentioned : (1) The myelinated fiber tracts of the 

 brain do not all mature at the same time, and fiber systems which are of 

 like function, that is, which are so connected as to perform special move- 

 ments in response to excitation, tend to mature at the same time. This is 

 Flechsig's "fundamental myelogenetic law," which may be stated in this 

 form, The myelination of the nerve-fibers of the developing brain follows 



