Clark, Comparative Anatomy of the Insttla. 91 



SUMMARY. 



1. Neither the insula nor the claustrum is constant among 

 mammals. 



2. The insula is present in the Primates, Carnivora, Pro- 

 boscidia, Ungulata and Cetacea. It is apparently absent in the 

 Cheiroptera, Insectivora, Rodentia, Sirenia, Marsupialia and 

 Monotremata. 



3. The claustrum may include parts of areas other than 

 the insula. 



4. The insula and the claustrum may generally be consid- 

 ered as parts of the same cortical area ; the claustrum may be 

 present without the insula ; both may be present or both may be 

 absent. 



5. The apparent absence of the claustrum when the in- 

 sula is present indicates that, relative to the surrounding areas, 

 the insular cortex is farther laterad than in those specimens 

 where it is so demarcated. The myelinic fibers of the sur- 

 rounding areas then pass wholly entad of the insular cortex. 



6. The insula is, in general, a convoluted area. 



7. The insula is sometimes a subarea but quite often it is 

 exposed upon the lateral surface of the cerebrum. 



8. The exposure of the insula may indicate either its own 

 excess or the deficiency of the overlapping adjacent parts. 



9. The gyres of the overlapping areas are always inter- 

 calated with those of the insula. 



10. Individual and lateral variations upon this area are 

 numerous and marked and are probably due in part to growth 

 in an enclosed space. 



11. The insula in the Primates, the Carnivora, some of 

 the Ungulata and Cetacea and perhaps others is a subarea or sub- 

 gyres at the bottom of the Sylvian fissure. 



12. The insula frequently is almost wholly within the Syl- 

 vian fossa. This condition is not believed to indicate in any 

 way a different relation to the parts entad but rather a more uni- 

 form development of the insula and the surrounding areas so 

 that no lateral overlapping has taken place and hence the ven- 

 tral end of the Sylvian fissure remains unclosed as a fossa. 



