CYSTS of the spleen are rare. Only 4 cases of cyst of the spleen have been seen at the Mayo Clinic from 1904 to 1934 (Pemberton'). Fowler2 (1940) was able to collect the records of 137 non-parasitic cysts of the spleen from the literature up to Jan. I, 1939. The first cyst of the spleen to be recorded was that of Andra13 in 1829. This cyst, discovered at a post-mortem examination, contained sebaceous material and hair, and these unusual findings present a pathological problem which is still unsolved. A sufficient number of cases of cysts of the spleen have now been recorded, and it is possible to draw some conclusions as to the different types of cysts which occur in the spleen. The classifification put forward by Fowler (1940) is widely quoted, but it is not an entirely satisfactory one in that it is based on unproven views as to the aetiology of some of the cysts. There are, however, certain broad distinctions which serve to group the cysts into main types which would be readily recognizable by all workers, and these will be considered in this paper.
The Echinococcus granulosus is the only parasite giving rise to cysts in the spleen. Arce4 (1941), of Buenos Aires, found the spleen affected by hydatid cysts in 3 per cent of his cases. He considers splenectomy to be the best treatment, as marsupialization is followed by a long-standing fistula leading to amyloid degeneration of the spleen, liver, and kidneys.
Multiple cysts of the spleen are occasionally seen in cases of polycystic disease of the kidneys, and such cysts, together with the multiple cysts of the liver and pancreas which are also sometimes encountered in this disease, present a problem which intensifies the difficulties to be faced in explaining the origin of the cysts of the kidneys. Indeed, no theory has yet been propounded which would satisfactorily account for cysts in all these viscera of such widely different structure and origin.
Of the other cysts of the spleen, classification is difficult in that many gradations of type are encountered, and intermediate types are not uncommon. These cysts fall into two main classes-large single cysts and multiple cysts. The single cysts may be associated with scattered small cysts, and the multiple cysts may be mainly made up of one or two very large cysts. The large single cyst is found in 80 per cent of the cases of non-parasitic splenic cysts (Fowler, 1940). These cysts are of three main types which shade one into the other. At one end of the scale is the dermoid cyst lined by stratified epithelium and containing sebaceous material and hair. Such cysts are excessively rare, and it is curious