THE DUCTLESS GLANDS GENERALLY. 741 



xanthin, salts of formic, acetic, succinic, and lactic acids, chloride of 

 potassium, arid alkaline and earthy phosphates. The bloodvessels of 

 the thymus are large and numerous ; the arteries penetrate to the 

 central cavity, and thence ramify towards the surface of the lobules ; 

 the capillaries traverse the soft white parenchyma in all directions, the 

 chief terminal plexuses being near the surface of the lobules ; the veins 

 are large and, what is unusual, do not accompany the arteries. The 

 lymphatics are also numerous and of great size, terminating, some in 

 the thoracic duct, others in the right lymphatic duct, and others 

 directly in the neighboring large veins. It is supposed that the 

 lymphatics assist in conveying the contents of the cavities of the 

 thymus into the blood ; but their direct communication with those 

 cavities has not been demonstrated. The nerves are small, and are 

 derived from the pneumogastric nerve, and the sympathetic system. 



The office of the thymus would seem to be, to prepare an albumi- 

 noid pabulum, fitted for the formation and maintenance of the blood, 

 exactly at that period of life when growth is relatively most rapid, i. e. 9 

 in the earliest years of infancy. It is possible, moreover, that its 

 nuclei and nucleated cells, especially those which resemble the lymph- 

 corpuscles, are the germs of future white blood-corpuscles, a view 

 especially urged by Hewson. The almost complete absence of fatty 

 matter, hydrocarbons, or carbhydrates, from the thymus, as well as 

 from the thyroid body and spleen, would seem opposed to the idea, 

 that any of these organs stored up such substances for the direct pur- 

 poses of combustion. Yet it has been conjectured that the fluid of 

 the thymus, forms a reserve of material suited for oxidation in the 

 respiratory process, at a time when such matters, derivable from the 

 waste of muscular tissue, are by no means abundant. (Simon.) Later, 

 however, in fully nourished children, the thymus becomes quite fatty, 

 its nucleated cells being converted into adipose cells, which might then 

 yield their fatty combustible matter to the blood. In the hibernating 

 Mammalia also, this organ continues to grow more rapidly than the 

 body, up to the adult period of life, and, when thus persistent, con- 

 tains much adipose matter. This is also said to be the case in most 

 Reptiles. It was at one time held, that the thymus body acted as a 

 diverticulum, in regard to the pulmonary circulation, in the child. 



The closed sacs of the tongue, tonsils, pharynx, stomach, and intes- 

 tinal canal. These, as elsewhere described (p. 604), whether solitary 

 or clustered, may be regarded as minute representatives of the larger 

 ductless 'glands, to which in their closed form, their vascularity, and 

 their albuminoid, granular, nuclear, and nucleated cell-contents, they 

 bear a certain generic resemblance. They might, indeed, be compared 

 to the Malpighian bodies of the spleen ; but they differ from the vesicles 

 of the thyroid body, in having no distinct cavity lined by an epithe- 

 lium. They might be said to stand in the same relation to the larger 

 ductless glands, that the small and simple tubular glands of the stomach 

 and intestine do to the large secreting glands, with extensive excretory 

 ducts. 



The ductless or vascular glands considered generally. When the 

 structure of these organs was less understood than it is at present, they 



