338 CLASS XVI. 



villi wanting, as in many singing-birds, wlien tliey are replaced by 

 zigzag longitudinal folds sometimes forming rhomboidal meshes, 

 which indeed may be present simultaneously with villi, or in the 

 posterior part of the small intestine may take the place of the villi 

 which exist in the anterior part. 



The rectum ends with a muscular ring in the cloaca, a cavity 

 surrounded by strong circular muscular fibres, which, as in most of 

 the lizards, has a transverse external opening. Into this cavity also 

 the ureters open, and, in males, the efferent vessels; in female birds 

 the oviduct terminates at its left side. Also behind and between 

 the ureters, behind a projecting transverse fold in the cloaca, there 

 opens a sac which, after its discoverer Fabricius ab Aquapen- 

 DENTE, is named bursa Fahricii. This part is a blind sac, of which 

 the base is turned upwards and lies behind the rectum; it has a thin 

 layer of muscle with fibres crossing in various ways; the internal 

 or mucous membrane is thick, whitish, and presenting longitudinal, 

 projecting folds. This part often appears to decrease in older birds, 

 or at least to be developed less than might be expected from the size 

 which it has in young ones; it occurs in both sexes without re- 

 markable difference in the developments 



The liver is large^, and almost always deeply divided into two 



^ Hence the opinion falls to the ground that this part belongs to the system of the 

 organs of propagation, as, for instance, that of Fabeicids, who thought that in copu- 

 lation the sperm was received in this organ of the female bird, and was kept there for 

 the' successive impregnation of the eggs. Other writers suppose this part to be an 

 arrangement for secretion, and compare it with the glandular sacs which in some mam- 

 mals are situated near the anus; others see in this bladder a vesica urinaria. HusCHKE, 

 the latest writer who, as far as I know, has expressed an opinion respecting it, thinks 

 that the bursa is the bladder into which, during a certain period of development, the 

 excretory ducts of the primordial kidneys or corpoi-a wolfiana open, and compares it 

 in consequence to the vesica urinaria of fishes which is situated upon the rectum, 

 i. e. at the backside of it (see above, p. 37). The bursa Fahricii is to be seen figured 

 in R. De C4RAAF Opera omnia, Amstelsed. 1705, 8vo, Tab. xvi, described p. 243, in 

 Tannenberg Observ. circa partes genitales masculas avium, Gottingas, 1789, 4to, Tab. 

 III. fig. 2d &c. Compare also Geopfroy St.-Hilaire Mem. du Mus. ix. 1832, p. 397, 

 who regards the bursa as the excretory duct (?) of the Glandidce Coivpen, A. A. Ber- 

 THOLD in Nov. Act. Acad. Cws. Leap. Carol, xiv. 2, 1829, s. 903—918, and ^M. 

 HuscHKE de bursce Fabricii origine, Jens, 1838, 4to. 



2 In some birds the weight of the liver forms J^t^, in the peewit Jgth, and in 

 a saw-bill {Mergus albeUus) even ji^^th of that of the whole body. Tiedemann's Zoo- 

 logic, II. s. 492. 



