FOOD AND ITS UTILISATION 



T09 



membrane is much modified at the spot where they should 

 arise. They are minute in many birds, e.g. pigeons ; of 

 considerable size in many birds, e.g. ducks. They attain 

 to a length of 85 cm. in the male capercailzie. Sometimes 

 there is only one, as in some fish-eating birds ; usually 

 there are two ; there is a third caecum in many ducks and 

 birds of prey. This is continuous with the gut to start 

 with. But it has been shown by A. Lelievre and E. Retterer 

 (1910) that the glandular epithelium of the third caecum 

 diverges from that of open glands ; it gives rise to a mass 

 of reticular tissue produc- 

 ing fluid and corpuscles, 

 which pass into the blood. 

 In abnormal double-legged 

 ducks and fowls there 

 may be a third caecum — a 

 curious visceral duplication 

 accompanying that of the 

 part of the skeleton. 



The walls of the intes- 

 tine are composed of five 

 layers : — 



(i) An outer tunic of 

 connective tissue, partly 

 elastic ; 



(2) Circular smooth 

 muscle ; 



(3) Longitudinal smooth 

 muscle ; 



(4) A submucosa with blood-vessels, lymph-vessels, and 

 nerves imbedded in loose connective tissue ; and 



(5) The digestive and absorptive endodermic epithelium. 

 A villus is an ingrowth of (4) and (5). There is often 



an opening of a tubular digestive gland between one villus 

 and another. It is a little point of some interest that in 

 Mammals the longitudinal muscular layer is outside the 

 circular, which is enough to show that Mammals and 

 Birds are on difli"erent tacks of evolution. As regards 



Fic. 26. — Diagrammatic cross-section 

 through the intestine of the bird. 

 I, outer investment of connective 

 tissue ; 2, circular smooth muscle ; 

 3, longitudinal muscle; 4, connec- 

 tive tissue with blood-vessels, lymph 

 vessels, and nerves ; 5, digestive 

 absorptive epithelium, with villi into 

 which ingrowths of (4) also extend. 



