246 THE VITAL FUNCTIONS. 



pect that in this inquiry material assistance would be de- 

 rived from an accurate anatomical examination of the or- 

 gans by which the more remarkable secretions are formed; 

 yet, notwithstanding the most minute and careful scrutiny 

 of these organs, our knowledge of the mode in which they 

 are instrumental in effecting the operations which are there 

 conducted, has not in reality advanced a single step. To 

 add to our perplexity, we often see, on the one hand, parts, 

 to all appearance very differently organized, giving rise to 

 secretions of a similar nature; and, on the other hand, sub- 

 stances 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 mi- 

 nute 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 convey- 



* Sometimes the secreting organ appears to be entirely composed 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. Verte- 

 brated animals present us with numerous instances of glandular organs em- 

 ployed for special purposes of secretion: thus, in the eyes of fishes there ex- 

 ists a large vascular mass, which has been called the choroid gland y 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, appa- 

 rently 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. 



