THYMUS GLAND. 483 



mus connects the lower portion of the lateral lobes. It covers the second and third 

 tracheal rings and is about half an inch wide and one-third of an inch thick. From the 

 left side of the isthmus, and sometimes from the left lobe, is a portion projecting upward, 

 called the pyramid. The weight of the thyroid gland, according to Sappey, is from three 

 hundred and fifty to three hundred and eighty grains. It is usually stated by anatomical 

 writers that it is relatively larger in the foetus and in early life than in the adult ; but 

 Sappey, from his own researches, is disposed to believe that its weight, in proportion to 

 the weight of the adjacent organs, does not vary with age. It is a little larger and more 

 prominent in the female than in the male. 



Structure of the Thyroid Gland. The thyroid gland is covered with a thin but resisting 

 coat of ordinary fibrous tissue, which is loosely connected with the surrounding parts. From 

 the internal surface of this membrane, are numerous fibrous bands, or trabeculae, giving 

 off, as they pass through the gland, secondary trabeculae, and then subdividing until they 

 become of microscopic size. By this arrangement, the gland is divided up into communi- 

 cating cells, like a sponge. These bands are mingled with numerous small elastic fibres. 

 Throughout the substance of the gland, lodged in the meshes of the trabeculee, are numer- 

 ous rounded or ovoid closed vesicles, measuring from -fa to -fa of an inch. These are 

 formed of a structureless membrane, and they are lined by a single layer of pale, granular, 

 nucleated cells, from ^-^ to ^Vfr f an i ncn i n diameter. The layer of cells sometimes 

 lines the vesicle completely, sometimes it is incomplete, and sometimes it is wanting. 

 The contents of the vesicles are a clear, yellowish, slightly viscid, albuminoid fluid, with 

 a few granules, pale cells, and nuclei. Robin has described in these vesicles curiously- 

 shaped, translucent, feebly-retracting, colorless bodies which he has called sympexions; 

 but there is little known of their constitution or properties. The vesicles are arranged 

 in little collections or lobes, with the great veins passing between them. 



Vessels and Nerves. The blood-vessels of the thyroid gland are very numerous, this 

 organ being supplied by the superior and inferior thyroid arteries and sometimes by a 

 branch of the innominata. The arteries break up into a close capillary plexus, surround- 

 ing the vesicles with a rich net-work, but never penetrating their interior. The veins 

 are large, and, like the hepatic veins, they are so closely adherent to the surrounding 

 tissue that they do not collapse when cut across. The veins emerging from the gland 

 form a plexus over its surface and the surface of the trachea, and they then go to form 

 the superior, middle, and inferior thyroid veins. The nerves are derived from the pneu- 

 mogastric and the cervical sympathetic ganglia. The lymphatics are numerous but are 

 difficult to inject. The exact distribution of the nerves and the origin of the lymphatics 

 are not well understood. 



State of our Knowledge concerning the Functions of the Thyroid Gland. It is gen- 

 erally admitted that the thyroid gland may be removed from animals without interfering 

 with any of the vital functions ; and this, taken in connection with the fact that it is so 

 often diseased in the human subject without producing any general disturbance, shows 

 that its function cannot be very important. Nothing of importance has been learned 

 from a chemical analysis of its substance. The blood of the thyroid veins has been ana- 

 lyzed, but the changes in its composition in passing through the gland are slight and 

 indefinite. An instance is quoted by Longet of periodical enlargement of the gland in a 

 female during menstruation, but there is no evidence that this is of constant occurrence. 



Thymus Gland. 



The anatomy of the thymus assimilates it to the ductless glands, but its function, 

 whatever it may be, is confined to early life. In the adult the organ is wanting, traces, 

 only, of fibrous tissue, with a little fat, existing after puberty in the situation previously 

 occupied by this gland. As there never has been a plausible theory, even, of the func- 



