412 MANUAL OF HISTOLOGY. 



secretion- of the pancreas for a short time ; stimulation of its 

 central end does the same. On section of all the nerves going 

 to the gland, there is a paralytic flow of the pancreatic juice. 



Development. The pancreas appears very early in foetal 

 life, developing from a mass of mesoblastic tissue in the duo- 

 denal wall. It is probable that hypoblastic tissue from the 

 same region passes into it from the ducts. 



THE THYMUS GLAND. 



The thymus gland is an organ whose function is unknown ; 

 it may be classed, however, on account of its structure, with 

 the lymph-glands, its tissue being of the adenoid type. It is 

 loosely enclosed in a vascular connective- tissue capsule, which 

 sends septa and processes into the interior of the organ. These 

 divide it up into small lobules of the size of a pin's-head to 

 that of a pea. Within the lobules are the characteristic ele- 

 ments of the gland the follicles which are also known as the 

 acini, alveoli, or granules. Running spirally through each of 

 the two long lobes is a central band or canal, and, upon unrav- 

 elling the gland, the various lobules are seen to be arranged 

 about this. 



The fibrous capsule is made up chiefly of white connective 

 tissue, mingled with which there are fine elastic fibres and stel- 

 late connective-tissue cells. At a few of the nodal points of the 

 larger reticulating fibres are found peculiar cavities, lined with 

 fusiform cells and containing a few lymph-corpuscles. They 

 are probably connected with the lymphatics, and the fibrous 

 capsule seems, as a whole, to be slightly touched with a 

 lymphoid metamorphosis. The external surface of the cap- 

 sule is covered with a single layer of flat epithelial cells. Deep 

 in the capsule is a rich plexus of vessels, and scattered spar- 

 ingly through it are medullated nerve-fibres. 



The follicles. Enclosed in each of the lobules, and making 

 up its substance, are from ten to fifteen, or even fifty (Frey), 

 small, spherical or polyhedral bodies. These are the follicles 

 of the gland. They are from <fo to -^ mm. in diameter, and 

 resemble very much the follicles in the lymphatic glands and 

 Peyer's patches, but present a more embryonal aspect. They 

 are held closely together by the surrounding tissue, which 



