Some of these dressing stations taking in wounded under shell fire were located in shell-proof dugouts. At many points light narrow gauge railroads had been built which ran from the dressing stations right up to the trenches. On these railways little cars pushed by hand were used both for bringing out the wounded during a battle and for taking in food, water and other supplies. It is, of course, impossible to lay such railways in many parts of the lines where they would be exposed to direct observation by the enemy, but they are becoming more and more numerous as their value in saving time and labour in the "man handling" of food and trench supplies has been proved. At one of these dressing stations where the railway came right up to the shell proof dugouts fresh shell holes in the neighborhood testified to the fact that the work of the field ambulances is at times not unmixed with excitement. The cases which accumulate at the advanced dressing station are given further treatment if required, and are evacuated by motor ambulance, usually at night, as the road to the station is frequently under the enemy's observation, to the field ambulance proper where they are given further treatment or dressings as the necessity may be. From the field ambulance the sick and wounded are cleared by motor ambulance convoy to the casualty clearing station, or possibly in cases of tired or slightly shell-shocked officers and men, to the rest stations or convalescent hospitals, of which there are a number well behind the firing line. At the casualty clearing station the men are checked over, their wounds redressed, operations performed, and all the work done necessary to enable the men to be passed on to the base hospital by hospital train or barge. These clearing stations, of which there are usually three in a town, may keep certain serious cases for days until it is deemed advisable to send them on. While one clearing station is filling up and treating the patients, the other will be sending all possible treated cases down the line. From the base hospitals, which are near the sea, the men are forwarded as soon as advisable by hospital ships for distribution among the hospitals of England. While a battle is in progress the men pass through this system so rapidly that they may be wounded one morning and be in a hospital in England the next. The medical officer, of course, is attached to the battalion, and goes everywhere with it, and under him are a number of stretcher bearers who gather up the wounded. The advanced dressing station is merely an advanced party from the field ambulance which itself is divided into three sections, each of which may operate independently according to the nature of the country. Each ambulance is self-contained, having its own transport, and by using tents can work in an area which has no houses or other shelter.