80 ZOOLOGY. 



r/Hvnx, an composed of bloodvessels or lymphatics solely, 

 which, alter dividing and subdividing, again reunite. As 

 examples of the first may be cited the ovarian vesicles and tin- 

 adipose cellules ; of the second, the thyroid;* the thynms:t 

 the spleen (Fig. 24); and the mesenteric ganglions are th< 

 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 bcini: 

 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, jj 



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; 

 sometimes they also are alkaline, but much more strongly 

 than the blood ; whilst some are characterized by the presence 

 of matters not to be found elsewhere, such as urea, casein, 

 butter, &c. 



160. It is probable that in all cases the secreted matter 

 exists in the blood already formed. It was thought, for 

 example, that the urea found in urine must be formed by 

 and in the kidneys, since it could not be detected by chemical 

 analysis in the blood ; but if these organs be destroyed in a 



* 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. 



t The thynius 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 chest, 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 glucote, 

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

 sumed by the respiratory act . 



The bloodvessels distributed to the walls of the tubes and cavities form- 

 ing the secreting organs, never communicate directly with their internal 

 cavities. 



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

 minute cellules or utricles, of which the inner wall <>f the s- ( r.-tini: 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. 





