GLANDS. 477 



VASCULAR GLANDS. 



The vascular glands all agree with each other in the absence 

 of excretory ducts or channels for the discharge of their 

 secretion, a deficiency which involves as a necessary conse- 

 quence the reception of the secreted fluids by the blood- 

 vessels. 



From the differences in the size and structure of these 

 several glands, it is very doubtful whether any other resem- 

 blance besides that just pointed out exist between them, and 

 it is most probable that each has a separate purpose to fulfil 

 in the animal economy. 



THYMUS. 



The thymus, although usually spoken of and described as a 

 single organ, is in reality double, and consists of two distinct 

 glands united to each other in the middle line by cellular 

 tissue only. 



Each separate adult thymus is constituted of numerous, 

 probably some hundred follicles, which vary in size from a 

 pin's head to, in some cases, that of a pea, and the walls of 

 which are made up of mixed fibrous tissue. 



These follicles are usually more or less rounded, but some- 

 times polygonal in shape from pressure ; they are loosely held 

 together by fibrous tissue, and by the blood-vessels which 

 supply them ; they each contain in their interior a cavity 

 which is more or less filled with a milky fluid. (See Plate LXI. 

 jftff.S.) 



Several of these follicles open into each other and into a 

 common receptacle or " pouch," and this last again opens into 

 the internal cavity of the gland, the face of which is thickly 

 studded with similar openings. 



On its exterior each follicle may be seen, even without 

 injection, to be invested with a very beautiful plexus of blood- 

 vessels, represented in Plate LXI.^zy. 8. 



QQ 2 



