SECRETION. 347 



which line the cavities of the abdomen, the chest, 

 and the head, and which are also reflected in- 

 wards 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 struc- 

 ture 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 conveying the secreted fluid 

 to a more distant point. These cells, or follicles, 

 as they are termed, are generally employed for 

 the mucous secretions, and are often scattered 



* 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 

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

 large vascular mass, which has been called the choroid gland, 

 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 proceeding from the organ, and 

 opening on the inner surface of the air-bladder. 



