STRUCTURES AND PROCESSES OF OBSCURE NATURE. 465 



the kreatin in becoming urea sets free. But it is by no means clear that 

 kreatin is such a stage. 



The evidence as far as it goes tends to show that the metabolism of pro- 

 teid is very complex and varied, that a large number of nitrogen-holding 

 substances make a momentary appearance in the body, taking origin at this 

 or that step in the downward stairs of katabolic metabolism and changing 

 into something else at the next step, and that the presence in various parts 

 of the body and even in the urine, in small quantities, of so many varied 

 nitrogenous crystalline substances, forming a large part of what are known 

 as extractives, has to do with this varied metabolism. Possibly the transfor- 

 mations by which nitrogen thus passes downward take place to a certain 

 extent in such organs as the liver and the spleen, which are remarkably rich 

 in these extractives. 



ON SOME STRUCTURES AND PROCESSES OF OBSCURE NATURE. 



408. The thyroid body. Certain structures of obscure nature, but 

 probably connected in some way or other with some of the metabolic pro- 

 cesses in the body, are often spoken of under the undesirable name of " duct- 

 less glands." Such are the thyroid body or gland, the pituitary body, the 

 thymus, and the suprarenal capsules. These differ from each other so essen- 

 tially that the only plea which can be urged in favor of considering them 

 together is convenience and our ignorance of their respective functions. 



The thyroid body is the one of the group most deserving to be called a 

 gland, since it, like the lungs, arises as a two-lobed diverticulum from the 

 ventral surface of the anterior part of the alimentary canal, and at first, 

 like the lungs also, behaves as if it were about to become a double racemose 

 gland. The connection with the throat, however, which should have become 

 a duct, is soon obliterated, and the two lobes, united with each other by an 

 isthmus across the trachea, lose all traces of any branching ducts within 

 them, and become transformed into masses of isolated ductless alveoli bound 

 together with connective tissue. 



Hence, when a section is taken through a hardened and prepared lobe 

 of an adult thyroid, what is seen is a limiting capsule of connective tissue 

 sending into the interior numerous septa, which surround and separate from 

 each other round or oval spaces, the sections of the isolated alveoli. These 

 are of variable size, some being visible to the naked eye, and each is lined 

 by a single layer of low columnar or cubical nucleated cells resting on a 

 basement membrane, leaving a large cavity, which in fresh specimens is 

 filled with a glairy fluid. The cells present no special characters. 



The septa of connective tissue, fairly rich in elastic elements, but remark- 

 ably free from adipose tissue, contain 'numerous bloodvessels derived from 

 the superior and inferior thyroid arteries, the branches of which, relatively 

 large and frequently anastomosing, end for the most part in capillary net- 

 works round the alveoli ; from these capillaries and those of the septa the 

 blood is gathered into veins also relatively large, which, forming plexuses 

 on the surface of the organ, end in the superior middle and inferior thy- 

 roid veins. The thyroid body is thus furnished with an abundant supply 

 of blood. 



The septa also contain a very large number of lymphatic vessels, which, 

 both on the surface of the organ and along the septa, are arranged in plex- 

 uses of anastomosing trunks of considerable size. Small nodules of adenoid 

 tissue are also found in the septa. 



The nerves of the thyroid body are also abundant. They are, in man, 

 derived chiefly from the cervical "sympathetic nerve, passing off from the 



30 



