SECRETION GENERALLY. 751 



shorter or longer cervical cornua, or extensions upwards into the neck, as in 

 Quadrumana, Bats, Rodents, Solipeds, and especially in the Ruminants. In 

 the ox tribe, its cervical part extends up to the lower jaw, forming the neck 

 sweetbread, and, from the calf, is the part sold as the best sweetbread. This 

 organ is a conjoined bilateral mass ; its structure is lobulated and sacculated, 

 as in Man. It is largest in the young animal and disappears later in life. It 

 is said to become larger in hibernating animals. In Birds it is represented, 

 but only in the chick and young bird, by a small tube, having slight dilata- 

 tions upon it ; but it is sometimes divided off into sacs. In young Reptiles, 

 it has also been found, and likewise in the tadpoles of most Amphibia ; but 

 not, it is said, in the siren or proteus, in which the lungs are the least devel- 

 oped. In Fishes, the thymus is absent. 



In the Amphioxus, none of those ductless glands are found, not even the 

 spleen ; in this animal, no colored blood-corpusdes exist. 



In the Non-vertebrata, these glands and colored blood-corpuscles are equally 

 absent ; nor is any organ recognized in them, as being specially concerned in 

 the process of sanguification. The liver, however, is almost universally present, 

 and its glycogenic function has been detected in the snail ; and as it is a blood 

 gland, forming blood corpuscles in the Vertebrate embryo, it may suffice for 

 the wants of the Non-vertebrate organism in reference to the formation of blood. 

 Such organs as the spongy masses around the great veins in the Cephalopods, 

 may, perhaps, be concerned in sanguification. 



SECRETION. 



SECRETION IN GENERAL. 



Secretion (secernere, to separate) is the separation, by a gland or 

 membrane, of certain materials, in a more or less fluid state, from the 

 blood, and their escape, by means of proper ducts or openings, or from 

 a smooth membrane, on to the surface, or into the interior, of the body. 

 This general process is, however, divisible into secretion proper arid 

 excretion. In secretion proper, the products are formed by a nutritive 

 process, the result of a special attractive, selective, or assimilative 

 power, possessed by some epithelial structure ; and moreover, after 

 being discharged from the mouths or ducts of the glands, or from the 

 surface of membranes, they are used for certain purposes in the living 

 economy. In excretion, the educts are rather eliminated from the 

 blood through the agency of special structures, also epithelial ; and 

 they are henceforth cast out from the body as effete, useless, or even 

 injurious substances. 



Secretion may be performed by glands, or by membranes; but 

 excretion is always effected through the agency of glands. 



The secreting glands are the liver, pancreas, the salivary and lach- 

 rymal glands, the true mucous glands of the nose, mouth, fauces, 

 pharynx, oesophagus, the duodenum, the simple tubular glands of the 

 stomach and intestines, other minute glands associated with the ducts 

 of some of the larger glands, the sebaceous and Meibomian glands, 

 and lastly the mammary glands. The secreting membranes are the 

 mucous, serous, and synovial membranes. The excreting or excretory 

 glands are the kidneys, and the sweat glands of the skin ; to a certain 

 extent, the liver, and perhaps the intestinal tubuli, especially those of 

 the great intestine ; perhaps, also, the sebaceous cutaneous glands ; 



