The distinction between ordinary lichen and prurigo is really more a ques tion of names than of things. It may be observed that lichen is more gene rally grouped in patches. prurigo is more diffuse ; the itching of the former is comparatively slight, that of the latter intense and intolerable; as a neces sary consequence the skin is abraded by the nails. and a case of prurigo is always marked by scratches and bloody points. The cases in which the dis ease runs an acute coutee, and those in which it presents any tendency to ulceration and suppuration, are both commonly referred to lichen, the more ordinary chronic papular eruption is usually called .prurigo.
One or two varieties must be mentioned, not so much on account of their individual importance, as that their diagnosis is obscure. The lichen circum scriptus assumes a very complete circular form, which in common parlance brings it under the general classification of "ring-worm," a name which in cludes diseases by no means related to each other; to the student this appear ance is apt to suggest the idea of lepra or even herpes. With the latter it ought not to be confounded, because there is no secretion, no vesication, no crust; from the former it is distinguished by the circumstance that desqua mation is the principal feature of the one, is only an accidental occurrence in the other. In lepra large white scales surround a portion of skin which scarcely differs from that of health, in lichen circumscnptue the whole surface is rough, even though the edge be more elevated than the centre: the des quamation of the cuticle occurs as small fine scales, and is quite a subor dinate phenomenon ; the patches of lepra are large or numerous, of lichen smaller and solitary.
The same affection occurring in the scalp gives rise to what is very often called porrigo decalvans, a name as undefined as the vulgar epithet of ring worm. It is marked by the hair falling off in a circular patch, the surface being roughened and covered with minute scales; there is no vesication, sup potation, or ulceration. In this respect it differs from most other diseases which produce loss of hair, when there has been some previous severe affec tion of the scalp, and the patch of baldness only comes to be remarked when the skin has again recovered its natural condition: in that form of porrigo to which the name decalvane should be limited, the hair falls off in consequence of disease of the bulb, apparently caused by a parasitic fungus, the skin being left perfectly smooth and free from scurf.
In the lichen strophulue of childhood the papular character of the eruption is least defined. It consists of distinct spots scattered all over the body, but
especially the arms and legs, which are white sad elevated, and have a semi transparent appearance, almost exactly analogous to a vesicle: it is less to be distinguished by its aspect than by the fact that, with the exception of yeti celloid eruptions, there is no disease in which solitary vesicles are uniformly distributed ; they are either grouped together, or they affect certain localities more than others.
The prurigo pudendi again deserves notice from the occasional absence of all eruptive character together. It is no doubt often caused by want of cleanliness, by the presence of irritating secretions, of slight eczema, or some form of parasite about the roots of the hair ; but undoubtedly pruritus does exist without any of these causes, and it must then be regarded as sympathetic of internal irritation of the uterus, the bladder, or the rectum. The same remarks apply even more constantly to prurigo podieia, which is constantly associated with internal hemorrhoids and ascarides. If these be regarded as instances of a sympathetic or neuralgic character, it may be doubted whether, in a great number of cases, the same explanation might not be given, the appearance of eruption being really the effect of scratching : this is espe cially true of that form which is associated with a gouty habit.
Lichen and prurigo are generally distributed on the outward aspect of the limbs, and avoid the flexures of the joints. In this respect they especially differ from scabies, with which, not withstanding the great dissimilarity of the original lesion, they are sometimes confounded, because of their intolerable itching, and the change which is produced in their appearance by con stant scratching.
§ 3. Squamous Diseases.—The next class is one in which the cuticle is materially altered in its form and character. It does not desquamate accidentally in consequence of a casual inter ruption to the secretion, as in scarlatina or erysipelas; nor does the presence of a papule, as in the last class, cause the premature death, so to speak, of the small portion of cuticle which covers it; but the epidermis is secreted in some abnormal manner which leads to its agglomeration into scales of some size. In one form, ichthyosis, they remain attached, and acquire a horny hardness; in the others, lepra and psoriasis, they gradually become discon nected with the cutis, and fall off.