I79 6 HUMAN ANATOMY. 



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

 in d.amcter, varyingly disposed as continuous masses or imperfectly separated cords 

 ;uul alveoli The cells piLess round nuclei which contain chromatm reticula. The 

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

 in,, only in the umndLte vicinity d the larger blood-vessels and not between the 

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

 wide and nuim-n, us capillaries, the attenuated membrane ot the mtercellu ar honey- 

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



FIG. 1515- 



A 



\'. 



!.>. 



'*..' "*''' 



^V;-v-.. 



, ... ,,... ,,,,,,,.. .-. . bowinff difen I tyn d itructnre. <4, prindpal <11 wraogd M 



continuous mawes; /'. l.r-.kcn up into lob uk-s by vascular septa (v) ; C, disposed as acini, some of which 

 contain ' (.-{ft" Iff tsh.) 



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

 tions, 1 opinions differ as to their histological relations. Schaper* and others incline 

 to the view advanced by Samlstroem, 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 or-. ins colloid-containing alveoli being occasionally present. 



The arteries distributed to the parathyroids are derived 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 totusand 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 e..ntinne^ tor some years to enlarge in certain directions and to dwindle in 

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

 its prime it is modi lately firm and of a pinkish color ; later it becomes very friable 

 and re-einblrs fat and areolar tissue. 



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

 It a MiiTiiimded \>\ a fil>n>ns mosaic which sends prolongations among the 

 lobulet. It is situated l>ein-ath the upper part of the sternum, rising, when largest, 

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

 tionally I die diaphragm. The oi-an is thickest above, where it rests on the 

 peri< aidium. and defends in front of the latter in two flattened lobes, more or less 

 distin. t, \\hieh 14 row thinner and sometimes diverge below:. These are separated by 

 a layer of tibnn-> ti^ue which enters obliquely from the front in such a way that 

 ali .ye the left l..|,e overlaps the other. The lobes are -cm-rally of unequal si/e, the 

 left one beini; more often the lar-er. Sometimes the lobes are' fused, and there may 

 be a third one between them. Mich variations merely implying irregularities of the 

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



1 A . riti. .il review , ,f the pinions ..f the epithelial orpins derived from the pharyngeal 

 I* >in ' ii I'v Kolm :n Merkel and Bonnet's Kr-ehnisse, Bd. ix , 1899. 



'Ai.tnv < inikro. Anal. 11. Kntwick., Ikl. \l\i 



