400 Maine; agricui^tural jJxpe^riment station. 1912. 



and the isthmus is very sharply marked. As clearly shown in 

 figure 455, the real limit of the two regions appears as a 

 narrow line extending around the oviduct. However, for some 

 distance on either side of this the character of the longitudinal 

 ridges is quite different. 



In general appearance the isthmus resembles the albumen 

 portion except that the ridges are not quite so high and the 

 walls are more contracted forming a narrower portion of the 

 duct. The isthmus in the laying hen measures on the average, 

 about 12 cms. in length. The distinctive function of the isthmus 

 is to secrete the shell membrane. 



. The isthmus is continued without any clear line of demarca- 

 tion into the expanded portion known as the uterus or shell 

 gland, (fig. 456) As may be seen from figure 456, the charac- 

 ter of the ridges is quite different in the uterus. They are more 

 numerous and are greatly broken up by transverse and diagonal 

 folding. The walls of the uterus are not so thick as those of 

 the two last mentioned regions. The length of this organ is 

 also about 13 cms. ,The lower end is bounded by the large 

 sphincter muscle, , , , 



The vagina extends from this sphincter mu'scle to the cloacal 

 wall. The walls of the vagina are very muscular. The epithelial 

 folds are much smaller than in the preceding section. The 

 length of the vagina is between 12 and 13 centimeters. 



In a typical section of the oviduct wall, as for example the 

 albumen secreting portion, we can distinguish the following 

 seven layers of tissue (cf. fig. 462) : (i) covering the outside 

 of the duct there is a thin, serous, peritoneal membrane; (2) 

 immediately inside of this is a layer of longitudinal muscle 

 fibers. As was recently pointed out by Miss Curtis ('10) these 

 muscle fibers arise from and are continuous with the muscula- 

 ture of the dorsal and ventral ligaments of the oviduct. (3) 

 Inside of these muscles is a layer of connective tissue in which 

 the larger blood vessels are found. (4) Next there is a layer 

 of circular muscle fibers, and (5) inside of this an inner layer 

 of connective tissue. (6) In all parts of the oviduct except 

 the funnel and vagina there is next a thick layer of convoluted 

 tubular glandsj and (7) finally inside of this glandular layer is 

 the epithelium of the duct. 



