258 The Island of Reil [CHAP. 



Moreover, in spite of what Pascal says, we have incontestable evidence to the effect that 

 the cortex of the inferior frontal gyrus dominates the motor mechanism in speech, and if 

 the cortex of the insula shares this function, it should surely present a similarity of 

 structure, but this we can now state without a moment's hesitation is not the case ; indeed, 

 it is one of the many interesting revelations of cortical histology to see how the cell 

 lamination and fibre arrangement of the fronto-orbital operculum change to an altogether 

 different type almost the moment the insular plane is touched, and this may be upheld 

 as one of our strongest reasons for refusing to abandon our original belief in favour of 

 Pascal's tenet. 



With regard to the involvement of the sensory (auditory) factor in speech, I would 

 not state so emphatically that the posterior division of the insula plays no part in audition. 

 As I have already stated, " auditory " cells are to be found in that portion of the insula 

 immediately adjoining the most anterior of the transverse temporal gyri, and the rest of 

 this division exhibits an approximation of structure to that obtaining in the great expanse of 

 the temporal lobe which has remained unspecialised, so that although I should think it 

 extremely unlikely that this particular area of cortex is endowed with important func- 

 tional attributes, I cannot write as positively regarding it as I can about the cortex of the 

 anterior insula. 



To argue as Meynert did, that because the claustrum attains a higher development in 

 man than in any other mammal, therefore this structure and the covering insula have 

 an important bearing on the development of the faculty of speech, is inadmissible in the 

 face of our present knowledge of the cortical localisation and connections of the speech 

 mechanism, and as evidence to the effect that speech is dependent on the integrity of the 

 claustrum is wanting, we cannot suppose that in cases of insular lesion it is involvement 

 of this structure which occasions the speech disabilities. 



If then speech can be dismissed as being unrepresented in the insula the question is. 

 what function does it subserve ? Although, as I have repeatedly said, histological appearances 

 in themselves form an indifferent support for argument, still in the absence of stronger 

 evidence I venture to say that they are significant on two counts. 



In the first place, as the posterior insula is covered by a type of cortex resembling 

 the temporal more than any other, to it I would indefinitely assign auditory attributes. 



Secondly, the anterior insula having characters sui generis must possess a different and 

 possibly a special function, and a point arising for discussion is whether it is in any way 

 related to the sense of smell or taste. Glancing back at the phylogenesis of this region 

 it seems very difficult to say exactly what, parts in the brains of lower animals correspond 

 to the respective anterior and posterior insulae, but one notices several points of interest, 

 and the first is the relation which the insula bears to the fissura rhimca. Thus it has 

 been observed that the Sylvian fissure is at first a most unstable element, only becoming 

 fixed in higher mammals, and that the primitive sulcus which it is customary to call the 

 Sylvian is regarded as being of the nature of a kink mechanically produced by flexure 

 of the hemisphere in the process of serial development. But the point of importance is that 

 when present it springs invariably from the rhinic fissure, and there seems little doubt that 

 this kink or cleft becomes in man the lower end or notched stem of the Sylvian fissure 

 (I am afraid we cannot as yet go so far as to say that it is the homologue of the sulcus 



