84 ZOOLOGY. 



such as the kidneys and other glands of great importance : 

 some glands have a reservoir placed in the course of the 

 excretory duct, intended to permit of the accumulation of the 

 secretion, and its residence therein for a time. The gall- 

 bladder (Fig. 33) and the urinary-bladder (Pig. 62) are 

 pouches of this nature. 



157. The imperfect glands vary still more in their 

 mode of conformation. Some are composed of small closed 

 cells, isolated or agglomerated ; others, called vascular gan- 

 glias, are composed of bloodvessels or lymphatics solely, 

 which, after dividing and subdividing, again reunite. As 

 examples of the first may be cited the ovarian vesicles and 

 the adipose cellules; of the second, the thyroid,* the thymus,")" 

 the spleen (Fig. 32), and the mesenteric ganglias are the 

 examples ( 75). Their functions are unknown. J 



158. Nature of Secretion. The secreting organs are 

 always disposed in the form of membranes, one surface of 

 which is bathed by the nourishing fluid, the other being 

 free and forming the interior of the cavity ; this is the utri- 

 cular surface. This surface, then, performs, as it were, the 

 office of a filter, allowing only certain substances to pass from 

 the interior of the bloodvessels into the interior of the secre- 

 tory tube. 1 1 



159. The secretions differ from each other, and from 

 the blood, out of which they are formed, in containing sub- 

 stances in great abundance, of which but very small quantities 

 are to be detected in the blood : they may contain free acids, 

 whilst the blood from which they are drawn is alkaline ; 



* The thyroid is a spongy, ovoid, vascular mass, of a glandular appearance, 

 placed in the neck, and attached to the front of the trachea. Its enlarge- 

 ment constitutes bronchocele, and when aggravated, goitres. It is not present 

 in birds, reptiles, and fishes, and in animals still lower. 



f The thymus is a glandular-looking body, extremely developed in the 

 foetus, but which diminishes after birth, and generally altogether disappears. 

 It is situated in the fore and upper part of the ci^est, behind the sternum, 

 and between the mediastina. It lies partly on the "pericardium. 



J The recent experiments of M. Bernard seem to show that the liver, in 

 addition to its other functions, secretes a sugary substance, called qlycose, 

 which it pours into the blood, but which disappears soon after, probably con- 

 sumed by the respiratory act. 



The blood-vessels distributed to the walls of the tubes and cavities form- 

 ing the secreting organs never communicate directly with their internal 

 cavities. 



|| Kecent observations seem to show that the essential secreting organs are 

 minute cellules or utricles, of which the inner wall of the secreting tubes are 

 formed ; these little cells empty themselves first, or are thrown off into the 

 interior of the tubes, and are as rapidly restored or renewed. They form the 

 layer called epithelium, forming the inner layer of all mucous membranes. 



