SECRETION. 311 



Other hand, substances of very different properties 

 produced by organs, which, even in their minutest 

 details, appear to be identical in their structure. 

 Secretions are often found to be poured out from 

 smooth and membranous surfaces, such as those 

 which line the cavities of the abdomen, the chest, 

 and the head, and which are also reflected inwards 

 so as to invest the organs therein contained, as the 

 heart, the lungs, the stomach, the intestines, the 

 liver, and the brain.* In other instances, the 

 secreting membrane is thickly set with minute 

 processes, like the pile of velvet : these processes 

 are called villi, and their more obvious use, as far 

 as we can perceive, is to increase the surface from 

 which the secretion is prepared. At other times 

 we see an opposite kind of structure employed ; the 

 secreting surface being the internal lining of sacs 

 or cells, either opening at once into some larger 

 cavity, or prolonged into a tube, or duct, for con- 

 veying the secreted fluid to a more distant point. 

 These cells, or follicles, as they are termed, are 

 generally employed for the mucous secretions, and 



* Sometimes the secreting organ appears to be entirely com- 

 posed of a mass of vessels covered with a smooth membrane ; in 

 other cases, it appears to contain some additional material, or 

 parenchyma, as it is termed. Vertebrated animals present us with 

 numerous instances of glandular organs employed for special pur- 

 poses of secretion : thus, in the eyes of fishes there exists a large 

 vascular mass, which has been called the choroid (/land, and which is 

 supposed to be placed there for the purpose of replenishing some of 

 the humours of the eye, in proportion as they are wasted. Within 

 the air-bladder of several species of fishes there is found a vascular 

 organ, apparently serving to secrete the air with which the bladder 

 is filled; numerous ducts, filled with air, having been observed pro- 

 ceeding from the organ, and opening on the inner surface of the air- 

 bladder. 



