There is no disease which can be confounded with ichthyosis. Certain trades produce an unusual thickness, hardness, and dry ness of the cuticle, which may, in some degree, simulate it; but when occurring in parts of the skin not so exposed there cannot well be any mistake. Sometimes, indeed, on recovery from chronic eczema, the skin may for a time be hard and dry, but the history of the case sufficiently distinguishes the two disorders ; ichthyosis is a congenital malady.
There is no practical advantage in separating lepra from psoriasis. Some cases are certainly more obstinate than others; and in the text-books of skin diseases several varieties are re corded which depend in great measure on the duration and intensity of the disease ; its essence is the same, and in diagnosis it matters little which name is assigned. As a general rule, those cases in which healthy skin is surrounded by squamous portions, especially in an annular form, are called lepra, while those in which numerous small spots, or single larger patches, are wholly covered. by scales are called psoriasis; the crusts too, in the former are more adherent, and consequently 'larger and whiter than in the latter.
The greatest difficulty in recognizing the character of the eruption is expe rienced when the scales have been removed by a warm bath : the fresh cuticle underneath them presents a red shining aspect, which may for a moment be mistaken for chronic eczema. When it begins by a solitary patch it may be difficult to distinguish it from lichen cireumscriptus, especially on the hand or face, where constant washing removes the scales as soon as formed. The distinction rests on the principle already enunciated, that in the squamous diseases the cuticle is secreted in an unnatural condition, and consequently where the scale has been removed the skin looks red, and smooth. and shining, whereas in lichen the detachment of the cuticle is only caused by its nutrition being interfered with from the existence of papules, which give a certain degree of irregularity to the surface.
Like the previous class these diseases especially affect the outer sides of the. limbs, and avoid the flexures of the joints. They are not necessarily attended with itching, but if once irritated the itching sometimes becomes very intense. 'They are essentially chronic in character, and the history only shows that there has been a rough patch observed somewhere or other which has not received any attention till it has attained some size, or till the same eruption ham appeared elsewhere.
Pityriatis used to be classed as a squamous disease ; perhaps one of its varieties, pityriasis capitis, marked by a constant exces sive desquamation of the cuticle over the scalp, which falls as white powder when the hair is brushed, ought still to be so con sidered: it is nothing more than an excess of natural secretion, and can scarcely be classed among diseases of the skin. Pityriasis versicolor is now referred to the parasitic growths ; its most pro minent feature is the change of color over the parts affected.
§ 4. Vesicular this class we meet vrith cases of very varying intensity, which, according to the stage at which they are seen, may resemble squamous or pustular eruption& The reason for grouping them together is, that the primary ele ment in all is a vesicle, and the practical utility of such a classifi cation consists in this, that when such an origin can be traced, there is no difficulty in deciding to which of the vesicular disease; any case ought to be referred. The first inquiry, therefore, will be how long the disease has lasted, and how rt commenced; s,nd then search must be made for a vesicle in the early stage. If the first appearance of the disorder cannot be traced, we have to remember that the serum must either continue to ooze away, keeping the part constantly moist, or harden into a gum like crust ; or that it may dry up altogether, leaving small, round, dry scale& as the only remains of the vesicle; but on the other land, by exposure, the cutis may be irritated, and produce a purulent secretion, which forms crusts like those of the pustular eruptions. The two last alone c,an give rise to any difficulty in diagnosis, and they belong to one form of eruption—viz., eczema.
In this variety a number of vesicles are always found together, coalescing and forming a patch of varying size. It is distinguished from the other vesicular eruptions by their neither being disposed in regular groups, nor occurring singly. In its simple form the vesicles either constantly rise on an uninflamed surface, and gradually disappear, or the skin continues red and moist after they are, burst: in the former, the appearance of freih vesicles prevents bur referring the shrivelled and dry ones to any scaly eruption ; in the latter, the moistened surface prevents its being mistaken for erythema, or erysipelas, which is the name commonly applied to it by patients.