340 



COMPARATIVE ANATOMY 



and the division of labour is not so noticeable. Thus in the series 

 of existing Birds we can trace the course of the phylogenetic 

 differentiation of the organ. 



The small intestine is usually of considerable length and 

 becomes folded on itself to a greater or less degree ; it varies, 

 however, in form, relative length, and diameter. 



The straight large intestine opens into a cloaca, and differs as 

 to its relative diameter. The caecum is usually paired, and may 



Oe 



FIG. 250, A. DIAGRAM OF THE (ESOPHAGUS AND STOMACH or A BIRD. 

 DM , glandular stomach ; Ig, crop ; MD, duodenum ; MM, muscular stomach ; 



Oe, Oe l , O3sophagus. 



FIG. 250, B. GLANDULAR STOMACH AND GIZZARD OF Fulica atra. 

 S, tendrinous disc. (Other letters as in A. ) 



FIG. 250, C. TRANSVERSE SECTION THROUGH THE LATERAL PART OF THE GIZZARD 



OF Tetrao uroycdlus. 



DS, glandular layer ; L, lumen ; MS, muscular layer ; HP, keratinoid 

 triturating layer. 



be extremely long (Lamellirostres, Rasores, Ratitse). All kinds of 

 intermediate stages between this condition and an entire absence 

 of a caecum are to be met with. When largely developed, it must 

 have an important relation to digestion, as an increase of surface 

 of the mucous membrane is thus effected ; this increase may even 

 be carried further by each caecum being provided with a spiral fold 

 consisting of numerous turns, as in the Ostrich. 



This so-called bursa Fabricii is a structure peculiar to Birds, 

 and arises as a small, solid, epithelial outgrowth from the ecto- 



