1896.] INTESTINAL TBACX OF BIBDS. 139 



After it leaves the duodenum, the dorsal mesentery expands into 

 a gi'eat, almost circular, fold, with the middle mesenteric vein 

 running out to the yolk-sac in the centre of the fold. The gut is 

 suspended at the circumference of this circular fold, and, in the 

 simple type, is thrown into a number of corrugated folds around 

 the circumference, which closely resemble the corrugated folds 

 in the Alligator. At the posterior part of this circumferential 

 part of the gut is the point where the cceca are attached, and the 

 cceca run forward along the sides of the posterior part of this 

 loop. In a simple case such as in this young bird the edge of the 

 mesentery corresponding to its line of attachment, and represented 

 by a dotted line in the figure, passes directly into the edge of the 

 mesentery of the rectum. But in most fully grown birds the part 

 of the gut with the attached croca has been rotated under the 

 rectum, that is to say over it as seen in the diagram, until the 

 point of attachment of the caeca is brought close up to the 

 starting-point of the duodenum. Consequently, when the gut is 

 lying on the table with its primitive ventral side uppermost, the 

 rectum and the rectal vessel are covered along the greater part of 

 their length by the circular part of the gut. Pinally, individual 

 folds, from among the numerous small corrugated folds of the 

 circular loop of gut, increase enormously in length, and Dr. Gadow 

 has shown that the number of the loops that grove out, and the 

 mode in which they lie, folded over or under each other within the 

 body, are characteristic of avian groups. Where the folded loops 

 come in contact with each other, minor short circuitings take 

 place in the veins, and it occasionally happens, notably with 

 Parrots, that secondary sheets of connective tissue, usually con- 

 taining masses of fat, bind loops belonging to different parts of 

 the circular fold very closely together. But even in these cases, 

 and without difficvdty in most birds, these loops may be dissected 

 from each other, and the primitive circular loop of mesentery 

 becomes apparent and is seen to contain the median branch of the 

 mesenteric vein. The series of figures in this communication 

 exhibit the gut when this unfolding dissection has been per- 

 formed. 



The rectum, or last part of the gut, in the vast majority of 

 cases retains its primitive straight position, and is closely attached 

 to the dorsal wall of the body-cavity by the posterior part of the 

 primitive straight mesentery. The rectal vessel or posterior 

 mesenteric vessel runs in this. It leaves the common stem of the 

 portal vein very close to the anterior mesentery or duodenal vessel, 

 and runs backward to the cloaca. Just in front of the cloaca a large 

 median vessel leaves this and runs upward to the surface of the 

 kidneys. There it forks, and each fork, after receiving several 

 veins from the parietes, runs forward along the under surface of 

 the kidney. 



I shall now proceed to describe the deviations from this ground- 

 type so far as 1 have had the opportunity of following them in the 

 main groups of birds. The kaleidoscopic variety, in which the same 



