In what circumstances will a physician be called as a witness at a trial?

In People v. Schuyler, supra, a physician was called as a witness for the people, who was employed by the board of supervisors of the county as the jail physician, and as such had medical charge of all prisoners in the jail. The defendant was confined in the jail for six months and the physician examined the defendant at the request of both parties. It did not appear that the defendant was at any time sick during the six months, or that the witness was called to attend upon or prescribe for him as a physician. The physician testified, in answer to a hypothetical question as to the defendant's sanity and it was held that the evidence was competent even if the witness was influenced by seeing the defendant while in the jail.