THE TIRO-GENITAL SYSTEM 187 



The function of the isthmus is to secrete, or to form, the shell 

 membrane, the membrana testacea. 



The fourth division of the oviduct is the uterus, or the shell-gland 

 portion (Fig. 60, C). There is no clear line of demarcation between 

 the isthmus and shell-gland portion, the walls gradually expanding. 

 In this region the folds of mucous membrane become leaf-like and of 

 considerable length, extending into the lumen, thus affording a 

 greater cellular surface. The same coats of the duct are present 

 here as in other parts; but the outer longitudinal muscular layer is 

 thicker and possesses more strength. The quantity of connective 

 tissue is about the same. The glands are of a tubular type and the 

 same two varieties of epithelial cells are found here as elsewhere, 

 namely, the ciliated high columnar and the unicellular mucous 

 variety. 



In the active glandular cells of the shell-forming region the nuclei 

 are small, dark staining, and lie toward the center of the cell. The 

 chromatin granules and nucleoli take on a comparatively deep basic 

 stain, but they do not show the intense stain found in the albumin 

 and the isthmus region. The cytoplasm of the uterine tubular 

 glands does not present as heavy a granular appearance as that of 

 the albumin portion. These cells are diffusely granular, the granules 

 appearing of one size and taking the stain faintly. The function 

 of this portion is to secrete, or form, the hard calcareous covering 

 which has been described at the beginning of this section. 



The fifth division of the oviduct is the vagina. There is located, 

 at the juncture of the shell-gland portion with the vagina, a strong 

 sphincter muscle. The vagina is that constricted portion of the 

 oviduct extending from this muscle to the cloaca. The mucous 

 membrane forms low narrow folds with secondary folds, which 

 appear continuous with those of the shell-gland portion. The core 

 of these folds is composed of connective tissue. The vagina in 

 the hen of average size measures from 12 to 13 centimeters long 

 (Surface). The inner or circular muscular layer is well developed; 

 it is much thicker than in any other part of the oviduct. This 

 extra development gives the power necessary to successfully expel 

 the egg. The outer longitudinal layer is not so well developed; its 

 bundles are scattered throughout the connective-tissue layers. The 

 egg is caused to move along in the oviduct by a successive series of 

 contractions of the circular muscular fibers posterior to it. There 

 are no tubular glands in this portion, but a simple layer of high 



