Apparatus, in physiology, a group or collection of different organs, which are associated in the performance of some function in which each one bears a particular part. Thus, the heart, arteries, veins, and capillaries together constitute the circulatory apparatus. ' The bones, ligaments, tendons, and muscles of the limbs form the apparatus of locomotion. The mouth, teeth, tongue, stomach, and intestine, with the accessory glandular organs, are the digestive apparatus. An apparatus may include not only different organs, but also entire systems. Thus the circulatory apparatus comprises not only the heart, which is an organ by itself, but also the arterial system, the venous system, and the capillary system. All of these, however, are essential to the circulation of the blood, and each performs its own special part in the function.