THE THYMUS 565 



thyroid than in that of the lower mammals. Likewise the cystic ducts, 

 lined by columnar or ciliated columnar epithelium, which have been de- 

 scribed by Kohn (1897), though of frequent occurrence in the lower 

 mammals are rarely, if ever, found in the human parathyroid. 



The parathyroids and the thyroid are frequently in anatomical con- 

 tinuity; this area is characterized by transitional histologic conditions. 

 Moreover, af^er thyroidectomy the parathyroids may alter their structure 

 so as to resemble very closely thyroid tissue. The parathyroids are gen- 

 erally regarded as having a function absolutely essential to life. How- 

 ever, the experimental evidence touching this point is conflicting. But 

 in certain forms, e.g., cat and dog, extirpation produces tetany which 

 results in death. Vincent interprets the experimental data as indi- 

 cating that the thyroid and the parathyroids constitute parts of the 

 same apparatus, to which the pars intermedia of the hypophysis cerebri 

 may have to be added- as a third constituent. 



The connective tissue of the gland is of variable quantity. It forms 

 a thin but dense capsule; occasionally trabeculas extend inward and 

 partially outline indistinct lobules. In many instances a hilum trans- 

 mits the larger blood-vessels by means of vascular trabecula? which 

 radiate to all portions of the organ. A delicate fibrous or reticular 

 stroma invests the individual cells, or the cell groups, when these are 

 present. Occasionally the cells are so closely packed that the stroma is 

 scarcely demonstrable. 



The blood supply of the parathyroid is exceedingly rich. Ar- 

 teries enter from the capsule, or at the hilum, and rapidly break up 

 into a plexus of broad capillary or sinusoidal vessels which follow the 

 fibrous bands of the stroma and are in intimate relation with the epi- 

 thelium. They are collected into thin-walled venules which retrace 

 the course of the arteries. 



IV. THE THYMUS 



The THYMUS was formerly regarded as an organ of fetal and in- 

 fantile life, and was believed to attain its maximum development during 

 the second year of childhood. After this time it was thought to be- 

 come gradually replaced l>y nilipose tissue, its retrograde metamorphosis 

 reaching completion at about the age of puberty. However, Waldeyer 

 (1890) showed that the thymus may persist even in advanced age as 

 a body of considerable size, and apparently functional. The more recent 



