Of all the operations pertaining to the domain of Veterinary Surgery, without doubt the practitioner is most frequently called upon to perform--more especially in breeding districts--that of castration, the destruction or removal of the essential organs of generation in our domesticated animals. It is, however, not nearly so often resorted to for purely surgical reasons as for purposes closely related to questions of agricultural and industrial economy, by reason of its effect upon the individuals of the various species of animals subjected to it, in order to improve their value and increase their usefulness to mankind. And that this is its practical effect is no modern discovery. As far back in antiquity as seven centuries preceding the Christian era, it was known and practised upon various animals. Of this we may find ample historical proof in the writings of Roman, Greek, and Oriental authors, where specific mention appears of the various methods employed, including the processes of excision, of crushing and of tearing. Even the castration of females was known to our less remote ancestors, the Danes having in the sixteenth century performed it on sheep, swine, cows, and even mares. The spaying of cows, however, seems to have been forgotten about the beginning of the present century, and it was not until the year 1831 that Thomas Winn, of Natchez (Louisiana), and afterwards Levrat (of Lausanne), brought it to the attention of veterinarians, as a means for the improvement of the milky secretion in cows. The operation may be considered under two distinct heads. Under the first it is to be considered as one of _necessity_, as when performed with a therapeutic object in view; as, for example, when it constitutes one of the first steps involved in the surgical treatment of strangulated hernia, or of diseased conditions of the testicles or ovaries, such as orchitis, epididymitis, sarcocele, hydrocele, cyst, etc. In the other case it is resorted to for reasons of mere _fashion_ and _convenience_, and has for its object the production of such a modification of the general organism as shall increase the adaptedness of the animals subjected to it to the uses to which they are applied; when, of course, the economic becomes the paramount and exclusive reason for thus interfering with the obvious creative purpose. It is the operation as performed under this general heading that we shall now principally consider.