| OCR Text |
Show 1896.] INTESTINAL TRACT OF BIRDS. 139 After it leaves the duodenum, the dorsal mesentery expands into a great, 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 ca?ca are attached, and the cseca 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 ca?ca has been rotated under the rectum, that is to say over it as seen in the diagram, until the point of attachment of the ca?ca 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. Finally, 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 grow 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 containing masses of fat, bind loops belonging to different parts of the circular fold very closely together. But even in these cases, and without difficulty 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 performed. 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 n o w proceed to describe the deviations from this ground-type so far as I have had the opportunity of following them in the main groups of birds. The kaleidoscopic variety, in which the same |
