The dermis contains a complex vascular system arranged in three plexuses. Many structures are supplied retrogradely or have their own vascular and nervous plexuses, e.g., the hair follicles. The varying morphology of pathologic changes is due to certain pathologic processes being principally associated with a particular stage of the vascular system and consequently taking place largely in a particular layer of the skin.

Skin blood vessels are difficult to distinguish in histological sections. The major veins have special wall structures in various skin regions to meet the particular physiological requirements of say the arm or leg. Contraction of the venous wall may cause the tunica muscularis to appear thick so that the vessel looks like an artery to the inexperinced eye. In addition, the venous tunica muscularis in an elderly subject may be substantially thickened, especially in the lower leg. The distribution and structure of the vessels differ according to body region.

Pityriasis Rosea is an erythematosquamous dermatosis. It is often misdiagnosesd as mycosis. Questioning of patients often reveals that they have worn new articles of clothing, unwashed, some days or weeks before the onset of the condition. One wonders, however, whether one would not get much the same answers from any patient questioned in the same way. Pityriasis rosea generally begins insidiously with a nummular primary lesion (primary plaque), often at the side of the trunk. The lesion may, however, occur on other parts of the body. Each has a diameter of about 1 to 5 cm, is red, scaling, with a tendency to form annuli by healing of the center with brownish discoloration (pseudoatrophy).

Other  veneral disease i.e chancroid, lymphogranuloma venereum and granuloma inguinale will be touched on briefly. The primary lesion of ulcus mole consist of papulooustule, which rapidly breaks down into a painfull ulcer with a soft margin.

The tissue destruction may be progressive, producing deep ulceration on the penis with perforation of the urethra. The lesion may also spread serpiginiously. The lymph nodes are enlarged and painful, they may degenerate and break down, to form  fistulas to the surface. Incubation period three to five days.

Toxic epidermal necrolysis is often a severe form of bullous drug eruption, but not in all patient.Minor inflammatory changes of the transitional mucosae and the conjunctivae often appear ten to 14 days before the syndrome. Other prodromal symptoms include a burning sensation of the skin and transitional mucous membranes, and sometimes a sensation of heat.

These symptom are followed by erythema, and macular rashes resembling those of measles, scarlet fever, or drug eruptions. The erythema may begin in the skin folds. On these sites, flaccid, wrinkled blister develop, often with lightning rapidity, as in genuine pemphigus ; this is what gave rise to the old term of acute, malignant febrile pemphigus with extensive vesiculation.