54 DESCRIPTIONS OF PREPARATIONS. 



right, the other on the left side in the natural position of the organ. The two 

 tendinous spots are the centres whence radiate the musculi laterales which make 

 up the bulk of the organ. They are composed of smooth muscle fibres which in 

 transverse section appear to lie in columns, the fibres in each column being 

 connected to the fibres in the adjoining columns by short tendinous fibres. Two softer 

 muscular bundles lie one close to the entrance of the proventriculus, the other 

 at the opposite pole. These are the musculi intermedii. The mucous mem- 

 brane is glandular ; and the glands secrete the horny internal lining. This lining 

 is discoloured by the food, and it is continually worn away by the attrition of 

 stones, &c., swallowed with the food ; and it is continually formed anew by the action 

 of the glands. If it is stripped off by force, the attached surface appears as if 

 covered by very fine short villi, or processes which have been pulled out of the 

 gland-tubes. In sections of the gizzard these processes can be readily made 

 out dipping into the gland-tubes ; they are conical, more transparent, and ap- 

 parently softer than the superficial layers. Vertical lines, apparently formed either 

 by irregularities of structure or by imbedded cells, are traceable nearly through the 

 thickness of the horny layer. In many birds, e. g. flesh-eating birds, the muscular 

 walls of the gizzard are thin and its secreted lining soft and tenacious. The degree of 

 development of the muscles and the lining is closely connected with the character 

 of the food as was shown by Hunter's experiment of feeding a Sea-gull with 

 barley. The muscles then became at least double the thickness of those in a 

 Gull which had lived on fish. Cf. Catalogue of Physiological Series, Royal College 

 of Surgeons' Museum, i. p. 49, preps. 522 D, and 523. 



For the bile and pancreatic ducts, see description of Plate II. 



The two caeca appended to the commencement of the large intestine are 

 very small in the Pigeon a contrast to the long caeca of the common Fowl, 

 Pheasant, Grouse, &c. The large intestine is short and straight, as in all birds 

 except the Ostrich. The rectal aperture lies at the apex of a cloaca common to it 

 and the urogenital ducts. The rectal region of the cloaca is large and is separated by 

 an annular ridge, which in some birds is but feebly indicated, from a small middle 

 or urogenital chamber into which open the ureters and genital ducts on the 

 dorsal wall, the genital apertures externally to those of the ureters. An annular 

 fold always present separates the urogenital chamber from the third, outer or pos- 

 terior chamber, the external opening of which is guarded by a strong sphincter 

 muscle. An aperture on the dorsal wall of this outer chamber leads in young 

 Pigeons into the Bursa Fabricii an ovoid sac with a narrow neck lying dorsally 

 to the cloaca. In the Ratitae (? Apteryx) the urogenital chamber opens into the 

 Bursa owing to the fact that the neck of the latter is not constricted, and its 

 aperture is commensurate with the dorsal aspect of the outer chamber of the 

 cloaca. In Plotus anhinga (Darter), a Carinate, Garrod found a large aperture to 

 the Bursa, and Forbes has confirmed the fact. The Bursa commences to atrophy 

 in the Pigeon at the sixth, in the Fowl at the eighth, month according to Martin 

 Saint-Ange. There seems to be much variety in this respect among birds, and it 

 is possible that it occasionally persists. As a rule, however, its aperture closes, 

 its cavity is obliterated and its walls atrophied a more or less fibrous remnant 

 persisting. It is large in the embryo. Its function and homology are unknown : 

 its cavity contains only remnants of faeces or concretions, the origin of which is 



