DUCTLESS GLANDS. 361 



and inferior thyroid arteries, and sometimes a branch of the 

 innominata. The arteries break up into a close capillary 

 plexus, surrounding the vesicles with a rich net-work, but 

 never penetrating their interior. The veins are large, and, 

 like the hepatic veins, are so closely adherent to the sur- 

 rounding 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 then go to form 

 the superior, middle, and inferior thyroid veins. The nerves 

 are derived from the pneumogastric and the cervical sym- 

 pathetic 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 generally admitted that the thyroid 

 gland may be removed from animals without interfering 

 with any of the vital functions ; and this, taken in connec- 

 tion 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 im- 

 portance has been learned from a chemical analysis of its 

 substance. The blood of the thyroid veins has been analyzed 

 by Colin and Berthelot, but the changes in its composition 

 in passing through the gland are slight and indefinite. 1 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 



1 COLIN, Traite de physiologic comparee, Paris, 1856, tome ii., p. 479. 

 8 LONGET, Traite de physiologic, Paris, 1869, tome ii., p. 398. 



