24 



fixation (fig. 31, Ct. e."), but this in all probability can 

 take place in life since there is a plentiful supply of 

 muscles in the sub-epithelial layers. 



Immediately under the epithelium there is a 

 supporting membrane. This is very delicate and almost 

 indistinguishable under the greater part of the area of the 

 leaflets, but thickens considerably, so that it becomes 

 the outstanding feature in stained sections, near the 

 efferent margin (fig. 33, Sup. m.). It is this substance 

 that has been termed cartilage or chitin. It is clear, 

 however, that this supporting membrane must be con- 

 sidered in conjunction with the connective tissue on its 

 internal face, that is, bounding the blood cavity. This 

 layer is distinctly peculiar. Seen in transverse sections, 

 all that can be observed are a few nuclei with very little 

 surrounding cytoplasm (fig. 32, Ct. con.). If, however, 

 a section is cut very slightly tangential to the leaflet, it 

 may be possible to secure the layer itself. It then appears 

 made up of squamous cells, often with the corners drawn 

 out (fig. 34). If these cells touched, making up a definite 

 membrane, it would be difficult to distinguish them from 

 an endothelium. Instead of this, they are more or less 

 scattered, and at odd places there is a crowding together 

 to form islands of pseudo membrane. This tissue is thus 

 very characteristic. Now the supporting membrane is in 

 close contact with these cells, and moreover, in places it 

 is quite continuous with the matrix surrounding 

 them. This brings out strongly its resemblance to the 

 matrix of ordinary connective tissue both in structure and 

 in staining. Hence I have come to the conclusion that 

 the supporting structure, thickened at the efferent 

 margin, is really a connective tissue, free from cells or 

 fibres, and that the cells which have formed it occur on 

 its inner surface. 



