198 Royal Irish Acadenuj. 



tal, and the posterior horizontal, are formed by the meeting of the 

 contiguous lips of the four opercula — the temporal, the fi'onto-parietal, 

 the frontal, and the orbital. Sometimes the two anterior limbs are 

 fused into one by the absence of a frontal operculum ; sometimes 

 they have a common stem and assume a Y-shape ; this occurs when 

 the apes of the frontal operculum or pars triangularis does not meet 

 the temporal operculum. In consequence of a growth-antagonism 

 between the fronto-parietal and temporal opercula, the posterior limb 

 of the Sylvian fissure descends on the outer surface of the hemisphere 

 up to the ninth year of life. Under the opercula lies the island of 

 Eeil. There is a close correspondence, which is plainest in the foetal 

 cerebrum, between the furrows and the convolutions on the insula and 

 those on the outer surface of the hemisphere. It has three radial 

 furrows similar in position to, and contemporaneous in origin, respec- 

 tively, with three fissures of which I shall presently speak, namely, 

 the Eolandic, the inferior prsecentral sulcus, and the vertical portion 

 of the intra-parietal sulcus. It is a curious fact that in the develop- 

 ment of the sulci and gyri, the right insula is usually in advance of 

 the left. Before birth the anterior end of the island is very nearly 

 fixed in relation to the anterior end of the cerebrum, while the 

 posterior end moves rapidly towards the occipital pole. After birth 

 the posterior end remains fixed, while there is an oscillation in the 

 position of the anterior end, which at first approaches, and afterwards 

 moves back from, the anterior end of the cerebrum. 



Very interesting are the results arrived at by a comparison of the 

 fissures in the two sexes, and in the human and primate brain. Riid- 

 inger asserted that all the convolutions of the cerebrum in the female 

 foetus are backward in their growth as compared with those in the 

 brain of the male fcetus. Dr. Cunningham has not been able to satisfy 

 himself of this ; but, so far as relates to the insula, he recognises it as 

 true. In male adults, the antero-posterior length of the island is 

 relatively greater than in the female. The gyri and sulci on its surface 

 are very poorly developed in the anthropoids, the orang being nearest 

 to man in this respect. There is no frontal or orbital operculum in 

 those apes. This constitutes one of the most striking differences which 

 exist between the convolutionary arrangement of the ape and that of 

 man. The submerged portion of the insula in the ape corresponds in 

 reality with little more than the posterior two-thirds of the insula in 



