As secretion, exudation and infiltration are so much concerned in the diseases of the eye, I ask your attention, in a general way, to these subjects, not that anything new is offered, but as an introduction to the topic under consideration.
Secretion is a physiologic process, and consists in the elaboration from the blood, or other sources, of material which is discharged on the external or internal surfaces of the body. The material so separated and discharged is a secretion. Many of the secretions have special uses, which do not involve their discharge from the body. Others have no office to perform, but are separated from the blood to be discharged from the body; such may be termed excretions. Most secretions in a healthy body consist of certain well-known combinations, but when these combinations vary, or when new materials enter into their construction, the secretion may be considered abnormal.