. Principes de zoologie : toucher la structure, le développement, la distribution et l'arrangement naturel des races animales, vivantes et éteintes avec de nombreuses illustrations : partie 1, physiologie comparative : pour l'utilisation des écoles et des collèges. tofhe blood, un autre processus est également en cours, par lequel le sang est analysé, comme il l'était ; certains de ses constituants sont sélectionnés et ainsi combinés pour former des produits à des fins d'utilisation, tandis que d'autres parties qui sont devenues usuelousateur préjudiciable au système sont prises par différents organes, et expulsés sous différentes formes. Ce processus est termedSe

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. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. tofhe blood, another process is also going on, by which theblood is analyzed, as it were ; some of its constituents beingselected and so combined as to form products for usefulpurposes, while other portions of it which have become uselessor injurious to the system are taken up by different organs, and expelled in different forms. This process is termedSecretion. 262. The organs by which these operations are per-formed are much varied, consisting either of flat surfaces ormembranes, of minute simple sacs, or of delicate elongatedtubes, all lined with minute cells, called ejyithelium cells, which latter are the real agents in the process. Every sur-face of the body is covered by them, and they either dis-charge their products directly upon the surface, as on themucous mem.brane, or they unite in clusters and empty intoa common duct, and discharge by a single orifice, as is thecase with some of the intestinal glands, and of those fromwhich the peispiration issues upon the skin, (Fig. 94.). OF THE SECRETIONS. 127 263. In the igher animals, whereseparate organs for special purposes aremultiplied, numerous sacs and tubes areassembled into compact masses, calledglands. Some of tliese are of large size, such as the salivary glands, the kidneys, and Vie liver. In these, clusters of sacs -pj„ goopen into a common canal, and this canal unites with similar ones forming larger trunks, such as wefind in the salivary glands, (Fig. 93, ) and finally they alldischarge by a single duct. 264. By the organs of secretion, tv/o somewhat differentpurposes are effected, namely, fluids of a peculiar characterare selected from the blood, for important uses, such as thesaliva, tears, milk, &c., some of which differ but little intheir composition from tha

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