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DR. P. CHALMERS MITCHELL ON THE 



paired caeca, and of vestigial remains of a second caecum, through the Orders and Sub- 

 Orders of Mammals, is in harmony with the view that all Mammals are descended from 

 creatures with paired cseca, and that for some unknown reason there has been a 

 tendency throughout the group for one of the members of the pair to degenerate. 



Form and Function of the Cecum. 

 In analysing the facts known as to the condition of the cseca in Birds, I have been 

 able to show (15) that there is no definite invariable connection between the nature of 

 the food and the degree of development of the caeca. Whilst, on the whole, it may be 



Pig. 50. 



Diagrams of Mammalian Ileo-cteoal Region. 

 I. Primitive stage with, paired cajca. 

 II. Stage in which one ctecum has been lost. 

 C, Cs. CaBca. 



S.I. Cut end of small intestine. 



LA. Pree tubular aperture of small intestine to colon. 

 H. Cut end of colou. 



said that in herbivorous forms the caeca are generally large, and generally small in 

 insectivorous, frugivorous, carnivorous, and piscivorous forms, there are many exceptions 

 and difficulties in the way of accepting such a generalisation. Thus the Owls and the 

 Falconidae have a diet that is closely similar, and, none the less, the Owls have a pair 

 of very long caeca, whilst in the Falconidee these organs are very much reduced and 

 apparently functionless. The insectivorous and omnivorous Rollers, Motmots, and 

 Bee-eaters have a pair of large caeca, whilst in Passerine Birds of similar habit the 

 caeca are vestigial glandular nipples. It is impossible to doubt that family-history 

 dominates function in this matter. Certain families tend to retain the caeca ; others 



