Finally, as regards the position of the _medici_, who were attached to the cohorts as well as to the alae, a few special remarks seem necessary. On a British inscription one of these army doctors is described as _medicus ordinarius_[114], which would naturally mean that he served in the ranks, and a passage in the _Digest_ confirms this by ranking the _medici_ among the _immunes_.[115] On the other hand, M. Ulpius Sporus, who is described in an inscription erected by his freedmen at Ferentinum in Etruria as _medicus alae Indianae et tertiae Astorum_ (sic)[116], seems to be on rather a higher level, as also M. Rubrius Zosimus of Ostia, who was doctor to the Cohors IV Aquitanorum in Germania Superior in the second century.[117] Both these men are apparently Greeks, and can hardly have reached their regiments by the ordinary recruiting channels.[118] It has been noticed also that the _medici_ appear to have a special position in some inscriptions of the Praetorian cohorts.[119] Probably, then, one may infer two classes of _medici_, the common soldier who possessed some elementary qualifications (first aid and blood-letting) and was given the position of an _immunis_, and the fully-trained professional doctor who was attached to a regiment but held no actual military rank. It was probably to distinguish himself from the latter class that the _medicus_ of the Tungrian cohort added the word _ordinarius_ to his title. As regards the rate and method of promotion, and the order of precedence of the various posts within the two groups of _principales_ and _immunes_, we know practically nothing. There is nothing to show that it was customary to hold several posts in a regular order,[120] or to become an _immunis_ before entering the _principales_. It was doubtless usual for a man not to receive commissioned rank without first holding some subordinate post, but we do not know that any such preliminary qualification was essential.[121] Owing to the length of service promotion was probably not rapid, but on the other hand the number of posts available was very large. In an _ala quingenaria_, for example, there were 16 decurions, 34 _principales_, and probably over 100 _immunes_.[122] Thus every soldier must have felt confident of obtaining sooner or later a position of greater ease and profit, and this, together with the fact that the ladder of promotion led to commissioned rank, and even to the coveted legionary centurionate, must have increased the attractions of the profession.