256 The Island of Reil [CHAP. 



Comparing this distribution with Flechsig's findings the most important point noticed 

 is that my posterior area that be it noted containing large fibres seems to coincide with 

 his "primordial" posterior field, but I am unable to confirm his division of the anterior insula 

 into three distinct areas. 



CONDITION IN THE ANTHROPOID APE. 



Unfortunately my examination of the insula in the ape has not been perfect. As 

 pointed out in the chapter on methods it is essential to the accurate survey of any given 

 part that drawings or photographs of the surface morphology be first made, otherwise correct 

 orientation is impossible; and as my material was received in the hardened condition, I was 

 unable to expose and figure the insula in the desired manner without destroying much of 

 the valuable specimen. 



Therefore, having no guiding landmarks, my inspection is wanting in topographic 

 accuracy, and particularly I regret that a comparison with the human arrangement has been 

 rendered imperfect by the fact that I have been unable to identify with any degree of 

 certainty the simian sulcus centralis insulae. However, in spite of these deficiencies, one or 

 two points of interest have been observed. 



In the hinder part of the insula a large obliquely placed gyms is observed running 

 parallel with the transverse temporal gyri, this seems to be the large gyrus posterior secundus 

 peculiar to the higher ape, and it is important to notice that it presents a type of structure 

 of a less primitive character than that in the anterior insula, and a type which has temporal 

 features more distinctly marked than in man. This point can be referred to as significant 

 of the probability that the hinder part of the insula in both man and ape has the same 

 function as temporal cortex. 



In describing the " intermediate precentral " area it was maintained that the simian 

 fronto-orbital sulcus is the homologue of the anterior limiting sulcus of the insula, and that 

 statement now finds support in the discovery that the primitive cortex of the ape's anterior 

 insula is not wholly operculated as it is in man, but reaches forward to the sulcus first 

 named. 



Lastly, the lower part of the insula again exhibits rhinic characters. 



FUNCTIONS OF THE INSULA. 



Phylogenetically the pallium of the insula is an old creation ; in early mammals it 

 envelops a relatively large surface of the cerebral hemisphere 1 , not until the higher forms 

 are reached does it give indications of becoming operculated as it is in the human brain, 

 and even in the anthropoid family of apes its concealment is still imperfect in the frontal 

 direction. 



Interesting in affording an illustration of the progressive expansion of the neopallium, 

 an expansion which, as seen in the human brain, is universal, the wide extent of the 

 primitive insula is further instructive inasmuch as it suggests a parallelism between its 

 pallium and that of the olfactory region; and whatever the function of the insula may 



1 In most lower animals, according to the comparative anatomist, the snprasylvian, postsylvian and diagonal 

 sulci are the homologues of the human superior, posterior, and anterior limiting sulci of the insula, respectively. 

 In some orders, e.g. Canidae, no sulcus diagonalis is present and then the definition of the anterior insular limit 

 becomes arbitrary. The sulcus diagonalis is accepted as the equivalent of the fronto-orbital fissure of higher orders. 



