The first and second 24th (Wessex) Field Ambulances were Territorial Battalions and the man at the head of each Battalion was not only a high ranking-officer such as a Lieutenant -Colonel, he would have been at least a Doctor of Medicine. His 2/IC would have been a Captain or Major who was a Bachelor of Medicine. The work of the Battalion was roughly divided into two - medical and logistical and a glance at the casualty statistics reveals just how successful their care was. Wounded men were quickly passed progressively along a  carefully staged line of posts which ran from the front line all the way back to England if necessary and the chances of the wounded were hugely improved by the initial care they received immediately after injury, at the casualty clearing stations where they were assessed. 

A Field Ambulance was not a vehicle although vehicles were used in the transport of the wounded. It was the name of a base post to which men were taken for initial treatment after being injured. Each Field Ambulance was composed of 10 officers and 224 men and was divided into three sections, each of which could sustain service by itself  if necessary. Many tasks , such as driving ambulances, fumigating, delousing, provision of water and sanitation, cooking etc. were performed by men drawn from the Army Service Corps and if your ancestor served with the RAMC, don't be surprised if they also saw service with the RASC.