THE INEXCITABLE REGIONS OF THE CORTEX. 769 



and partly to the gyrus hippocampi ; the third (which appear from one 

 to several months after birth) go partly to the third frontal gyrus (by 

 way of the fasciculus subcallosus), partly into the frontal lobe, almost to 

 its apex ; not ending here, however, but bending sharply backwards to 

 the middle third of the gyrus fornicatus, to the anterior part of the first 

 frontal gyrus, and to the foot of the second frontal gyrus. Almost all 

 these fibres come from the lateral nucleus of the thalamus opticus, 

 which constitutes, in Flechsig's opinion, a special station on the course 

 of the afferent tracts towards the cortex, and which receives from below 

 the chief part of the fillet layer, the superior peduncle of the cerebellum, 

 the longitudinal bundles of the reticular formation, and part of the 

 posterior longitudinal bundle. 1 It appears obvious that these three 

 systems, which are developed at considerable intervals from one another, 

 cannot be regarded as having an identity of function, although it is not 

 easy, in the present state of our knowledge, to indicate exactly what 

 their several purposes may be. The fibres of the first system being 

 distributed almost exclusively to the motor regions of the cortex, and 

 becoming developed very early (a month before birth), may perhaps be 

 regarded as excito-motor fibres for cerebral reflexes ; they perhaps 

 represent the fibres which convey impressions of muscular sense. Those 

 of the second system, which are developed at about the time of birth, 

 may well be those for the conveyance of cutaneous perceptions ; their 

 existence affords strong confirmation of the view that the limbic lobe 

 is intimately connected with this sensory function. 2 It is still more 

 difficult to conjecture the special function of the third system of 

 Flechsig, which is developed so much later than the others, and the 

 myelination of which apparently extends over a considerable period of 

 time, but it may perhaps be constituted merely by reinforcing fibres for 

 the other two systems. 3 



The Inexcitable Eegions of the Cortex. Association Areas 



of Flechsig. 



It will be seen that if we mark off upon the surface of the cerebrum 

 the areas from which a response to stimulation is obtainable, and to 

 which we can with some probability ascribe a definitely localised function, 

 whether motor or sensory, a large proportion — relatively much larger in 

 man than in monkey, 4 and becoming progressively smaller in descending 

 the scale of intelligence in mammals — remains over. With regard to 

 those portions of the surface which for the sake of brevity we may 



1 According to Flechsig and Hosel, part of the main or upper fillet goes directly into 

 the internal capsule and is connected with the central gyri, hut this statement has not been 

 confirmed by any other observer. Mott, after lesions of the fillet in the monkey, could 

 trace no degeneration beyond the thalamus {Brain, London, 1895, vol. xviii. p. 1). 



2 There are also a certain number of published clinical cases which tend to favour the 

 view that the limbic lobe, and especially the gyrus fornicatus, is especially related to 

 tactile sensibility (see Savill, Brain, London, 1891, vol. xiv. p. 270, and ibid., vol. xv. 

 p. 448), but the importance of this evidence is diminished by the fact that extensive lesions 

 of the gyrus fornicatus, which do not involve important neighbouring structures, are 

 practically unknown. 



3 Flechsig appears to regard all these systems of fibres as connected with cutaneous 

 sensibility, and considers that their distribution indicates exactly the extent of the 

 area for cutaneous sensibility, which he accordingly locates in the several regions to which 

 they pass, namely, the Rolandic region (including the two central gyri, the gyrus marginalis, 

 and the bases of the first, second, and third frontal gyri), the gyrus fornicatus, cornu 

 ammonis, and part of the gyrus hippocampi. 



4 Amounting, according to Flechsig, to two-thirds of the whole cortex in man. 



VOL. II. 49 



