_Elements to be Proved in an Action for Service, etc.—General Advice._—The result of these rules may be thus summarized, viz.: The elements to be established in an action for services by a physician against a patient are three in number—(1) the employment; (2) the performance of the services;[178] and (3) the value of the services, that value being either a fixed value determined by the terms of the contract between the parties, or the reasonable worth and value of the services determined by evidence of experts upon that subject. It is, therefore, important that physicians and surgeons should be advised, when entering upon the practice of their profession, to keep a record of their transactions and of their business generally. Because, if they are compelled to go into court to recover for their services, they will be called upon to describe with minuteness the character and extent of the services they have performed in order that the value thereof may be correctly ascertained and determined in the suit. Any person in active practice who is not blessed with a most tenacious and particular memory is liable to forget a great many details which, with a record in hand, properly kept, could be brought to his memory and be testified to with absolute truthfulness and conviction. And the record itself, when properly shown to be a book of original entry, is generally receivable in evidence, as a memorandum of the transaction.[179] CHAPTER V. OF THE PRIVILEGES AND DUTIES OF PHYSICIANS AND SURGEONS WHEN SUMMONED AS EXPERT WITNESSES IN COURTS OF JUSTICE.[180]