Oesophageal Valve 65 



intima gives the appearance of a deep chitinous band, 

 encircling the base of the valve externally, and best seen 

 in fresh specimens from which the epithelium of the 

 cardiac chamber has been removed. 



' 2. The epithelium is difficult to observe, but it closely 

 follows the course of the chitinous intima. It consists of 

 polygonal, nucleated cells, which decrease in size towards 

 the upper bend. When it is in an inactive condition the 

 nuclei of the epithelium are relatively very small. 



' 3 .It is not clear whether the hasement-memhrane 

 continues to the upper bend, or, as seems more probable, 

 disappears in the region of the lower bend. 



' 4. The muscular layer is reflected for about half the 

 length of the valve. The boundaries of the muscle-cells 

 become faint, and their thickness gradually diminishes as 

 the reflected layer passes upwards. 



' Between the muscular layer and its reflected con- 

 tinuation is a blood-space similar to that already described 

 in the Simulium larva. During the passage of large 

 masses of food along the lower part of the oesophagus 

 the inner tube may be so greatly distended as to obliterate 

 the space and squeeze out the blood (fig. 46). A number 

 of oblique fibres may be seen to pass from the inner 

 to the reflected muscular layer across the lower half 

 of the blood- space. These fibres, which are probably of 

 connective tissue, bind the walls of the fold together, but 

 so loosely as to admit of considerable relative movement. 

 It is obvious that fibres passing directly across would be 

 much shorter, and would restrain the movements within 

 much narrower limits. The fibres at their inner ends 

 seem to be attached directly to the walls of the oesopha- 

 geal muscle-cells, but at their outer extremities they are 

 attached to a more or less cylindrical layer of connective 

 tissue, which forms the outer boundary of the blood-space ; 

 it is generally applied to the surface of the reflected 



