179^ 



HUMAN ANATOMY. 



The gland-tissue consists of closely placed polygonal epithelial cells, about .010 mm. 

 in diameter, \'aryingly disposed as continuous masses or imperfectly separated cords 

 and alveoli. The cells possess round nuclei which contain chromatin reticula. The 

 cells are surrounded by a honey-comb of delicate membranes, fibrous tissue appear- 

 ing only in the immediate vicinity of the larger blood-vessels and not between the 

 epithelial elements. The latter lie against the endothelial lining of the relatively 

 wide and numerous capillaries, the attenuated membrane of the intercellular honey- 

 comb alone intervening. While admitting the independence of the parathyroids as 



Fig 





Sections of human parathyroid bodies, showing different types of structure. .(. i>iiiiciiial cells arranged as 

 uniform continuous masses; />', broken up into lobules by vascular septa (v) ; C, disposed as acini, some of which 

 contain colloid (f). < 200. {After H'e/sk.) 



distinct organs, as now established by both anatomical and physiological investiga- 

 tions,' opinions differ as to their histological relations. Schaper^ and others incline 

 to the view advanced by Sandstroem, that the parathyroids correspond in structure 

 to the immature and undeveloped thyroid. Welsh, on the contrary, denies this 

 resemblance and points out the close similarity to the anterior lobe of the pituitary 

 body, in both organs colloid-containing alveoli being occasionally present. 



The arteries distributed to the parathyroids are derix'ed from the branches sup-, 

 plying the thyroid body. Regarding the lymphatics and the nerves little is known ; 

 the latter are chiefly sympathetic fibres destined for the walls of the blood-vessels. 



THE THYMUS BODY. 



The thymus is apparently an organ of service to the nutrition — possibly blood- 

 formation — of the foetus and infant, since it usually reaches its greatest size at about 

 the end of the second year, having grown since birth fairly in proportion to the 

 body. It continues for some years to enlarge in certain directions and to dwindle in 

 others ; coincidently deposits of fat appear and it gradually degenerates.. When in 

 its prime it is moderately firm and of a pinkish color ; later it becomes \-ery friable 

 and resembles fat and areolar tissue. 



Shape and Relations. — The appearance of the thymus is that of a glandular 

 organ. It is surrounded by a fihroiis caps7(le which sends prolongations among the 

 lobules. It is situated beneath the upper part of the sternum, rising, when largest, 

 perhaps 2 cm. into the neck, descending to about the fourth costal cartilages, excep- 

 tionally as far as the diaphragm. The organ is thickest above, where it rests on the 

 pericardium, and descends in front of the latter in two flattened lobes, more or less 

 distinct, which grow thinner and sometimes diverge below. These are separated by 

 a layer of fibrous tissue which enters obliquely from the front in such a way that 

 above the left lobe overlaps the other. The lobes are generally of unequal size, the 

 left one being more often the larger. Sometimes the lobes are fused, and there may 

 be a third one between them, such variations merely implying irregularities of the 

 fibrous septa. The thymus lies in front of and above the pericardium, and against 



' A critical review of the relation.s of the ei)ithelial organs derived from the pharyngeal 

 pouches is given by Kohn in Merkel and Bonnet's Ergebnisse, Bd. i.x., 1S99. 

 ^Archivf. rnikro. Anat. 11. Eiitwick., Bd. .xhi., 1895. 



