OF THE SECRETIONS. 127 



263. In the higher animals, where 

 separate organs for special purposes are 

 multiplied, numerous sacs and tubes are 

 assembled into compact masses, called 

 glands. Some of these are of large size, 

 such as the salivary glands, the kidneys, 



and the liver. In these, clusters of sacs "pv" "s" 



open into a common canal, and this canal 

 unites with similar ones forming larger trunks, such as we 

 find in the salivary glands, (Fig. 93,) and finally they all 

 discharge by a single duct. 



264. By the organs of secretion, two somewhat different 

 purposes are effected, namely, fluids of a peculiar character 

 are selected from the blood, for important uses, such as the 

 saliva, tears, milk, &c., some of which differ but little in 

 their composition from that of the blood itself, and might 

 be retained in the blood with impunity ; or, the fluids 

 selected are such as are positively injurious, and cannot 

 remain in the blood without soon destroying life. These 

 latter are usually termed EXCRETIONS. 



tt 



265. As the weight of the body, except during its period 

 of active growth, remains nearly uniform, it follows that it 

 must daily lose as much as it receives ; in other words, the 

 excretions must equal in amount the food and drink taken, 

 with the exception of the small proportion discharged by the 

 alimentary canal. Some of the most important of these 

 outlets will be now indicated. 



266. We have already seen (37) that all animal tissues 

 admit of being traversed by liquids and gases. This mutual 

 transmission of fluids from one side of a membrane to the 

 other is termed endosmosis and exosmosis, or imbibition and 

 transudation, and is a mechanical, rather than a vital, phe- 

 nomenon, inasmuch as it takes place in dead as well as in 



