Pathological Anatomy. - When dealing with their classification we gave in outline the main pathological changes to be met with in corns. It now only remains to give the same matter in slightly greater detail.
In dry corn the changes we meet with are those accompanying blood extravasation. From excessive compression of the parts, or from the effects of direct injury, a portion of the sensitive sole has become lacerated. The escaping blood stains the surrounding soft tissues after the manner of blood extravasation elsewhere. If the escape of blood is sufficiently large, the horn fibres in the immediate vicinity also are stained. It is this stain in the horn that is the direct evidence of the injury, and is itself popularly known as the corn. It may vary in size from quite a small spot to a broad patch as large as half a crown, while its colour may be a uniform red, or a mottled red and white. The microscopic changes in this connection are illustrated in Fig. 99.
Fig. 99. - Horizontal Section Of A Corn. The Section Cut At About The Base Of The Papillae Of The Sensitive Sole. A, Papillae, With Horn-Cells Surrounding Them; B, Interpapillary Or Intertubular Horn; C, Hollow Spaces In The Intertubular Material Filled With Blood; D, A Papilla And Its Surrounding Horn-Cells Filled With Blood.
Ordinarily, this ecchymosis of the horny sole is due to injury of the sensitive sole immediately beneath it. It may, however, proceed from injury to the vessels of the laminae either of the bars or of the wall. In this case the ecchymosis of the horny sole may be explained by the fact that the escaped blood tends to gravitate to that position.
When the corn is of long standing, or is due to repeated injuries on the same spot, the horn adjacent to the lesion becomes hard and dry, and often abnormally brittle, simply on account of the inflammatory changes thus kept in continuation. This is often seen when attempts are made to pare out the corn with the knife.
Should the injury be seated in the sensitive laminae, then the brittle nature of the horn secreted by the injured tissues makes itself apparent by the appearance of cracks in the wall of the quarter. Why this should occur will be readily understood by a reference to Fig. 100.
Fig. 100. - Inner Surface Of The Wall Of The Quarter, Showing Changes In The Horny Laminae Brought About By Chronic Corn.
It will here be seen that the injury to the keratogenous membrane has led to great interference with the secretion of horn from the sensitive laminae. As a result, the regularly leaf-like arrangement of the horny laminae has been largely broken up. Certain of the laminae are altogether wanting, while others are broken in their length and rendered incomplete. With this condition there is always more or less contraction of the quarter.
Microscopic examination of the structures involved in such a case reveals the fact that with the contraction is an alteration in the normal direction of the horny and sensitive laminae.
They become bent backward, and, instead of the regular and normal arrangement depicted in Fig. 32, show the distorted appearance given in Fig. 101.
From the appearances and characters of the blood-stain in the horny sole we are able to deduce evidence relative to the duration and nature of the injury.
Fig. 101. - Perpendicular Section Of The Wall Of A Contracted Quarter In A Case Of Chronic Corn. Both The Sensitive And Horny Laminae Are Bent Backwards, And HaeMorrhages Have Taken Place At The Base Of The Sensitive Laminae.
When, for instance, the stain is not to be found in the superficial layers of the sole, but is only discoverable by deep paring, then the injury is a recent one.
Where the stain is met with in the superficial layers of horn, and is quickly pared out, then the injury has been inflicted some time before, and has not been repeated. When, as is sometimes the case, layers of horn that are stained are found alternated with layers that are healthy, then we have evidence that the cause of the corn, whatever it may be, is not in constant operation.
Similar indication of the age of the injury is also afforded by the colour of the lesion.
A stain that is deep red is proof that the injury is comparatively recent.
A distinct yellow or greenish tinge, on the other hand, is evidence that the injury is an old one.
In the Moist Corn we have, in addition to the blood extravasation, the outpouring of the inflammatory exudate. In the most superficial layer of the horn this may not be noticeable. As one cuts deeper into the sole with the knife, however, it will be found that the lower layers of horn are more or less infiltrated with the discharge. This gives to the horn a soft consistence, a yellow appearance, and a touch that is moist to the fingers.
With the accompanying inflammation the cells in the neighbourhood of the injury are enfeebled and their normal functions interfered with. We may thus expect a corresponding interference with the growth of horn. This is exactly what happens, and as one cuts deeper still into the horn a point is finally reached when a well-marked cavity is encountered. A pale yellow and usually watery exudate fills it. This cavity points out the exact spot where the force of the injury has been greatest, where death of certain cells of the keratogenous membrane has resulted, and where the natural formation of horn has for a time been suspended.