5 o 4 TEXT-BOOK OF PHYSIOLOGY. 



second later than when the frontal lobe was stimulated ; and that as 

 the motor reaction takes place after extirpation of the frontal region, 

 the route of the efferent impulse cannot be to and through the 

 frontal lobe, but probably through some lower center. The same 

 facts hold true for the reactions of the ear-muscles following stimu- 

 lation of the temporal lobe. 



The view that the cortex of the cerebrum can be divided into 

 separate and independent though physiologically related motor and 

 sensor areas has been questioned in recent years, and a somewhat 

 different interpretation given to the facts. It is believed by many 

 physiologists and neurologists that the so-called motor and sensor 

 areas are so closely related that it is almost impossible to distinguish 

 one from the other either anatomically or physiologically. Thus the 

 Rolandic region is believed to be both motor and sensor in function, 

 the former, however, being more predominant in the pre-cehtral, 

 the latter in the post-central, convolution. As these two functions 

 are so intimately blended and their anatomic substrata so difficult 

 of separation, it is thought the term sensori-motor should be em- 

 ployed as more descriptive and more in accordance with the facts 

 to the entire Rolandic region. 



This view has been strengthened by the results of the embryo- 

 logic investigations of Flechsig, which show that different nerve-tracts 

 become medullated or receive their myelin investment at successively 

 later periods and that the tracts which first become myelinated and 

 are hence first functionally active, belong to the afferent system. 

 Among the first to undergo myelinization are three tracts numbered 

 by Flechsig i, 2, and 3, which arise largely from the median nucleus 

 of the thalamus and the medial lemniscus and pass to the anterior 

 and posterior convolutions, to the para-central lobule and foot of 

 the superior frontal convolution, and to the foot of the third frontal 

 convolution respectively. It is these fibers which convey nerve im- 

 pulses to the cortex and furnish information regarding changes taking 

 place in the body itself and thus lead to the performance of muscle 

 movements. This area is therefore primarily a sensor area, an area 

 for body- feelings, cutaneous, tactile, muscle, and visceral, and second- 

 arily a motor area. The afferent fibers to this region become mye- 

 linated during the ninth month of intra-uterine life, the efferent fibers 

 from it become myelinated during the third month of extra-uterine 

 life. 



By the same method of reasoning the gustatory, olfactory, audi- 

 tory, and visual sense areas are to be regarded as sensori-motor in 

 character, for embryologic investigations show that subsequently 

 to the myelinization of the afferent tracts* connecting the sense-organs 

 with the cortex, efferent nerve-tracts arise from or near to the 

 same centers and undergo myelinization. In other words, these 



