ORGANS OF SECRETION. 575 



mentary forms of the most complicated glands in the highest 

 classes, are merely simple co3ca prolonged from the surface 

 on which they open. The transition from secreting lining 

 or investing membranes, to secreting glands, is gradual and 

 almost imperceptible, and the most complicated glands of 

 the highest animals, as the liver and the pancreas, scarcely 

 merit the name of glands in their first stages of development, 

 or in the lowest tribes of animals. 



As digestion is the most important function of organic 

 life, and the digestive organs the most universal in animals, 

 the largest, the most numerous, and the most important 

 glands of the body, are developed from, and communicate 

 with the interior of the alimentary canal. The duct of a 

 gland is the gland itself, which may be a simple follicle, or a 

 tubulus ramified so as to compose a conglomerate mass, like 

 the liver ; but this membranous duct, with its vessels and 

 nerves, appear to be alone, though mysteriously, concerned 

 in the production of the secretion. Not only are the liver, 

 the pancreas, the salivary, and other chylopoietic glands, but 

 ducts developed from the digestive canal, and minutely 

 ramified towards their closed extremities, but even the 

 urinary, the genital, and the pulmonary organs themselves, 

 may be regarded as developments of the same kind. As the 

 digestive canal is not only lined with a secreting membrane, 

 continued through all its glands, but is also provided with a 

 muscular tunic to move its contents; so we observe this fibrous 

 contractile coat continued along the ducts of glands, to an 

 indefinite extent, to expel the secretions from their interior. 



In the first development of glands, the part from which 

 they are to originate, becomes thickened, soft, and highly 

 vascular, and constitutes a blastema or nidus through which 

 the growing ducts are to ramify. The blastema somewhat 

 indicates the form of the future gland, by its early division 

 into corresponding lobes and lobules ; and it becomes gra- 

 dually absorbed, as the developing tubuli ramify through its 

 substance. The glands which first make their appearance 

 in the lowest animals, are those most important in the eco- 

 nomy, those most constant in all the higher classes, and 

 which attain the greatest magnitude and complication in 

 quadrupeds and man ; and there are many considerable 

 temporary glands in the embryos of the highest animals, 



