14 Investigations on the Life-History 



strongly with the distended stomach and gullet of the salmon taken in 

 the Baltic and North Sea. His results are described on p. 165 et seq, 



It seemed desirable to re-examine the subject, and especially to 

 subject the digestive tract of the salmon under various conditions to 

 minute histological investigation. 



THE STOMACH OF SALMON ENTERING THE RIVER. 



The organ is surrounded by a serous coat, while beneath this are two 

 layers of non-striped muscle. The external layer is thin and is disposed 

 longitudinally ; the internal layer is circular and five or six times as 

 thick as the outer one. Between the two layers run blood vessels, 

 lymphatics, and a nerve plexus, with an unusually large number of nerve 

 cells, often grouped together into comparatively large ganglia. Beneath 

 the muscular layers lies the submucosa, with many large blood vessels. 

 The muscularis mucosse consists of two layers, an external longi- 

 tudinal and more delicate, and an internal circular rather thicker layer. 

 The mucosa may be divided into two parts, the glandular portion and the 

 connective tissue portion which underlies the glands and which supports 

 them. The mass of connective tissue underlying the glands is very 

 considerable, being often nearly as thick as the glandular layer-, and is 

 itself divided into two layers by the remarkable structure described by 

 Oppel in the stomach of the trout, the " membrana compacta " or 

 " stratum coinpactum." This layer, in whatever plane the stomach is 

 cut, is always found as a compact hyaline band lying rather nearer the 

 muscularis rnucosre than it does to the fundi of the glands. It is, of 

 course, pierced by the blood vessels, etc., but I have never seen muscle 

 strands from the mu<cularis mucosse passing through it. It contains no 

 nuclei, and no structure can be made out in it by ordinary methods. 

 Nuclei lie upon it, however, and the fibres of the connective tissue on 

 either side are directly continuous with it (Fig. 10), and behave to all 

 reagents in the same way as it does. It is, therefore, certainly of con- 

 nective tissue origin, and is as certainly not elastic tissue. The fibrous 

 tissue outside this layer is more delicate than that inside it, and the 

 inner layer contains more blood vessels, especially large venous spaces, 

 than the outer one. A special feature of both layers is the great 

 number of large eosinophile leucocytes to be found in "the meshes of the 

 connective tissue ; in fact, nil the many leucocytes present here seem to 

 be of this variety (Fig. 10). 



In the epithelial lining of the mucous membrane the cells are of three 

 types : 



1. The superficial epithelium and that forming the ducts of the 



glands. 



2. The intermediate epithelium of the glands. 



3. The zymin-forming epithelium of the glands. 



1. The superficial epithelium (Fig. 8) is precisely like that found 

 in most fish stomachs long cylindrical or columnar cells with oval nuclei 

 about the middle of the cell. Where they are best developed, on the 

 projections between the mouths of the glands, they have a narrow clear 

 hem on the surface, and below that a portion of the cell, which looks 

 more hyaline and stains rather more deeply than the rest, and is sharply 

 marked off from the deeper portion, which shows an ordinary proto- 

 plasmic structure. These cells are not goblet-cells, and no such cells 

 are found in the stomach. The glands are very thickly set over the 

 stomach and open by wide ducts, down which the superficial epithelium 

 extends unaltered for some distance, usually down to the point where 

 the glands begin to branch. Oppel notes that this branching or opening 



