742 SPECIAL PHYSIOLOGY. 



were sometimes supposed to possess parts analogous to the terminal 

 acini, vesicles, or dilated ends of the ducts of true secreting glands; 

 and the absence of the ducts themselves was said to form the most 

 marked distinction .between them and these glands. But the thyroid 

 body alone has distinct vesicles, limited by a membrana propria and 

 an epithelium, and so far approximating to the characters exhibited 

 by the commencing ducts of a secreting gland. In the spleen, and 

 even in the suprarenal bodies, the inter-trabecular areolse, and the 

 columnar loculi, are not so surrounded, but are mere interspaces in an 

 areolar framework. The minute encapsuled Malpighian bodies of the 

 spleen, and likewise the closed sacs of the alimentary canal, have no 

 lining epithelium or true basement-membrane. The branching saccu- 

 lated canals, and secondary cavities, or acini of the thymus, cannot 

 be compared to true glandular structures, for they also are destitute 

 of a lining membrane and epithelium. Indeed these ductless glands, 

 instead of resembling the secreting glands with ducts, possess char- 

 acters approximating them rather to the lymphatic glands, with their 

 numerous loculi and albuminoid corpuscular contents; but they differ 

 in this, that their cavities do not open directly into the lymphatic 

 vessels. 



Considered generally, their proper parenchyma, with its granular 

 plasma, nuclei, and nucleated cells in various stages of growth, consti- 

 tutes their most important and characteristic anatomical element. The 

 rest of their structure is either the framework of the organ, or con- 

 sists of the bloodvessels, lymphatics, and nerves. 



The physiological influence of these organs in the economy must be 

 exercised on the blood, and must be exerted, especially through a nu- 

 tritive process, by the nuclear and nucleated cell -like constituents. 

 The blood entering such an organ yields to it, by exudation through 

 the walls of the capillaries, a common plasma, from which, by a nutri- 

 tive process dependent on the special attractive, selective, and assimi- 

 lative powers of the microscopic elements, certain special materials are 

 separated. The residue of the plasma re-enters the circulation, either 

 directly through venous, or indirectly through lymphatic absorption, 

 as in every instance of simple nutrition. Hence, in the first place, 

 the blood which passes through these organs must be modified, as in 

 all nutrition, by the abstraction of certain of its constituents; and the 

 effect is peculiar in each organ. 



But, secondly, the proper substance of these ductless glands cannot 

 remain unchanged and inactive, subject to no further metamorphoses, 

 and productive of no special influence upon, or service in, the economy. 

 On the contrary, it would seem certain, that something must also be 

 added, by their agency, to the blood as it passes through them. The 

 materials attracted from the blood by their proper substance, and 

 elaborated within them by a sort of nutrient secretive act, are returned, 

 more or less altered, into the blood current. This may chiefly be 

 accomplished by solution and venous absorption in the spleen, supra- 

 renal bodies, and thyroid body, or by lymphatic absorption in the 

 thymus and closed sacs of the alimentary mucous membrane, or by 



