Although of rare occurrence calculi are now and again met with in the course of the salivary ducts, and more especially in that one which conveys the saliva from the parotid gland into the mouth. Salivary calculi are of a grayish-white hue, very dense, and rough on one surface. When existing alone they are rounded or elongated, but when two or more occur together in the same duct they present a more or less irregular form owing to the friction they exercise upon each other during the movement of the jaws in mastication. Some specimens are very small while others (fig. 95) reach several ounces in weight and four or more inches in length. The longer calculi are usually somewhat concave on the side nearest to the lower border of the jaw over which the duct passes.
Chemically they are found to consist of carbonate and phosphate of lime and a small quantity of animal matter. Carbonate of lime is the chief constituent, and forms from eighty to ninety per cent of the whole.
The formation of these stones is provoked by any obstructive cause which prevents the escape and favours the accumulation of salivary fluid in the duct, or by the entrance into it of small particles of grit or other foreign matter such as hay, straw, etc, upon which the salts contained in the fluid will gather in a succession of superimposed layers.
It is not until the stone or stones have reached a considerable size that salivary calculi are recognized. Attention is first directed to them by a small bulging on the lower border of the jaw, or the groom's hand is brought into contact with them when the horse is being cleaned. When of large size a considerable enlargement appears on the site of the growth, which on manipulation is found to be sharply circumscribed and movable on the jaw.
In some instances the skin and underlying tissue around the growth are considerably thickened as the result of irritation provoked by blows. The escape of the salivary fluid being interfered with, it is made to collect and distend the duct along its course towards the gland.
Where the stone is discovered while still small an attempt may be made to force it upward along the duct and out into the mouth, but this will need the greatest care and most gentle manipulation. Large stones, which cannot be so expelled, must be removed bodily.
Preliminary to an operation the hair must be removed either by close clipping or shaving, the skin is then thoroughly washed and brushed with soap and warm water, and afterwards dried and irrigated with a five-percent solution of carbolic acid. An incision is now made through the skin, and the duct laid open either transversely or longitudinally, just so far as to allow the removal of the stone. The divided edges of the duct are then carefully brought together with catgut sutures, and having thoroughly cleansed the wound from blood, and freely irrigated it with carbolic solution, the edges of the skin are brought together and the part covered with sterilized wool. After the operation is completed the patient should be tied in pillar reins and kept without solid food for from forty-eight to fifty-six hours.
Fig. 95. - Two Views of a Salivary Calculus. This calculus measured four inches in length.