EXTRA-VASCULAR TISSUES.
Epithelium.— On cicatrising and on fistulous surfaces, on the inner wall, or amid the con. tents, of cysts, as a coating for hsematomata, and as a lining for new vessels, tessellated epithelium occurs as a purely adventitious product. The retained and accumulating epidermis forming corns and callosities, or that thrown off in excess from the skin in pi tiriasis, or front the genito-urinary mucous surface in various states of disease, or from the intestinal surface in cholera, &c., can only be viewed as products of supersecretion. Hy pertrophy of the papillae of the skin, with excess of epidermal formation (Univ. Coll. Mus.), a state prone to give rise to obstinate ulceration, cannot fairly be considered under the present head. Perhaps the epithelium accumulated in cutaneous sacs produced by dilatation and ocdusion of sebaceous follicles may be considered adventitious. So likewise are those productions, elongated or flat. known as " horns," and which are essentially com posed of epidermis. Commonly springing from a dilated and diseased sebaceous follicle, and mixed abundantly with fat, slightly with saline matter, the basis of the future "horn" is at first soft, subsequently becomes inspissated and hard, when its increasing dimensions, carrying it beyond the limits of the follicle, place it under the influence of the atmosphere. Layer upon layer of epidermis continues to accumulate at the surface of the follicle, and eventually a conical mass, some inches in length, may be the result, Horny-looking productions sometimes form on ulcerated surfaces, simple or cancerous.
Pseudo-tumours, composed essentially of epithelium, and susceptible of vascularization, form, it is affirmed, on sonic mucous surlitces, —the uterine for instance. We have not
met with productions of this kind.
Nail.— (See TOOTH, p.
Cartilage.—Adventitious cartilage, at one time believed to take rank among the most common, is now known to be one of the rarest of new formations : the microscope has certainly dispelled a cloud of' error on this subject, by simply showing that cartilagI. nous-looking products are not necessarily cartilaginous. Such products are most com monly composed of dense fibrous substance, or induration-matter,— as in the instance of so called loose cartilages of joints.* Adventitious cartilage is either of the em bryonic or adult type : the former has already been described under the name of enchon drotna. Cartilage of adult type, certainly, sometimes forms the matrix in which adventi tious bone originates ; this we have seen beautifully exemplified in spicular osteophytes. Analogy would lead to the admission that a cartilagtnous stage should always precede bone-production ; yet not only is proof of the constancy of this stage wanting, but we have looked in vain for its traces in many spechnens of adventitious bone. In a very few prepar ations of that rare variety of false-joint in which a pseudo-synovial membrane is pro duced, the bony surfaces (whether from frac ture or dislocation) have exhibited a cartila ginous look in patch-work ; but we have not had an opportunity of submitting such a speci men to microscopical examination. Of the appearance of cartilage in various Growths enough has already been said.
Nor is the production of cartilage for reparative purposes more easy. A fractured cartilage unites by dense fibrous tissue, or by bony substance.