1911 Encyclopædia Britannica/Posse Comitatus

POSSE COMITATUS (Lat. “power of a county”), a summons to every male in the county, between the ages of fifteen and twenty, to be ready and apparelled, at the command of the sheriff and the cry of the county, to maintain peace and pursue felons. Ecclesiastical persons, peers and such as laboured under any infirmity were not compellable to attend. Owing to the establishment of county police, the sheriff does not now pursue felons, but by the Sheriffs Act (1887, sec. 8, sub-sec. 2) the calling out of the posse comitatus is expressly authorized if the sheriff finds any resistance in the execution of a writ. In view of the sheriff’s duty to raise, if necessary, the posse comitatus it is no answer by him, for non-execution of a writ, to say that he was resisted.

See P. E. Mather, Sheriff Law.