SECRETION. 207 



the healthy condition of the animal. In the last experiment it is said that 

 the normal secreting tubules of the gland undergo atrophy. 



We must believe from these experiments that the pancreas-prediiees a sub- 

 stance of some kind which is given off to the blood or lymph and which is 

 either necessary for the normal consumption of sugar in the body, or else, as 

 is held by some, 1 normally restrains the output of sugar from the liver and 

 other sugar-producing tissues of the body. What this material is and how it 

 acts has not yet been determined satisfactorily. It is interesting and sugges- 

 tive to state in this connection that post-mortem examination in cases of dia- 

 betes mellitus in the human being has shown that this disease is associated in 

 some instances with obvious alterations in the structure of the pancreas. 



The Thyroid Body. The thyroids are glandular structures found in 

 all the vertebrates. In the mammalia they lie on either side of the trachea 

 at its junction with the larynx. In man they are united across the front of 

 the trachea by a narrow band or isthmus, and hence are sometimes spoken 

 of as one structure, the thyroid body. In some of the lower mammals 

 (e. g. dog) the isthmus is often absent. The thyroids in man are small 

 bodies measuring about 50 millimeters in length by 30 millimeters in width ; 

 they have a distinct glandular structure but possess no ducts. Histological 

 examination shows that they are composed of a number of closed vesicles vary- 

 ing in size. Each vesicle is lined by a single layer of cuboidal epithelium, 

 while its interior is filled by a homogeneous glairy liquid, the colloid substance 

 which is found also in the tissue between the vesicles lying in the lymph- 

 spaces. This colloid substance is regarded as a secretion from the epithelial 

 cells of the vesicles, and Biondi, 2 Langendorff, 3 and Hiirthle 4 claim to have 

 followed the development of the secretion in the epithelial cells by micro- 

 chemical reactions. While the interpretation of the microscopical appearances 

 given by these authors is not identical, they agree in believing that the colloid 

 material is formed within some or all of the epithelial cells, and is eliminated 

 into the lumen with or without a disintegration of the cell-substance. More- 

 over, Langendorff and Bioudi believe that the colloid material is finally dis- 

 charged into the lymphatics by the rupture of the vesicles. The composition 

 of the colloid is incompletely known. 



Parathyroids. The parathyroids are a pair of small bodies lying lateral or 

 posterior to the thyroids, and in some animals (rat) they are apparently con- 

 tained within the substance of the thyroids. They are quite unlike the thyroids 

 in structure, consisting of solid masses or columns of epithelial-like cells which 

 are not arranged to form acinous vesicles. According to Schaper 5 these bodies 

 are not always paired, but may have a multiple origin extending along the 

 common carotid in the neighborhood of the thyroids. Experimental investi- 

 gations seem to show that these bodies are probably immature structures 



1 See Kaufmann : Archives de Physiologic normale et pathologique, 1895, p. 210. 



2 Berliner Ktinische Wochenschrift, 1888. 3 Archivfur Physiologie, 1889, Suppl. Bd. 

 4 Pfluger's Archivfur die gesammte Physiologie, 1894, Bd. Ivi. S. 1. 



* Archivfur mikroskopische Anatomie, 1895, Bd. xlvi. S. 500. 



