ORGANS OF THE BODY. 469 



still more so the submucous tissue, is very abundantly supplied with 

 absorbent canals, while the muscular substance is only traversed by 

 regular vessels. In the roots also of the filiform papillae is to be found a 

 capillary network, from which caecal prolongations are sent off into the 

 papillae themselves. 



The development of the tongue in the embryo commences as early as 

 the sixth week of intrauterine life, in the form of a thick ridge, which 

 seems subsequently to become stationary as regards its growth. The 

 papillae are said to be rudimentarily formed in the third month. 



REMARKS. 1. Compare Todd and Bowman, vol. i. p. 437-2. Much variety is to 

 be seen in the form of the filiform papillae. It is not uncommon also to meet with a 

 thread-like fungus, the leptothrix buccalis, in great quantity among and upon these 

 papillae. 



249. 



Behind tho foramen caecum the mucous membrane presents to the 

 unaided eye a more or less smooth appearance. Here the laminated 

 epithelial stratum merely covers a series of small simple papillae, each 

 supplied by a single vascular loop. 



In this locality a number of different varieties of secreting organs make 

 their appearance. In the first place, even anterior to the foramen caecum, 

 small scattered mucous glands present themselves, which form more 

 posteriorly under the circumvallate papillae, and towards the root of the 

 tongue a thick continuous glandular layer. 



On the under surface of the tongue also, near its tip, are to be found 

 two other racemose glandular masses of considerable magnitude. These 

 empty themselves by several ducts at either side of the fraenum (Blandin, 

 Nukn). Their functions are, however, still unknown. 



From the posterior fourth of the tongue on, finally, the tissue of the 

 mucous membrane, commences to undergo at points a lymplioid metamor- 

 phosis. This may be absent in many mammals, but attains in others, 

 on the other hand, as for instance in the pig, a high degree of develop- 

 ment. In the latter animal this process may advance to the formation of 

 follicles in the larger papillae imbedded in a densely reticulated connec- 

 tive substance (Schmidt). 



This metamorphosis of the mucous tissue (by which the pharynx also 

 may be affected) leads, as it advances, to the formation of larger and more 

 sharply defined lymphoid organs, varying greatly as regards distribution 

 and structure. They are largely met with among the mammals, and are 

 not absent in man. 



Among these may be numbered the lingual follicles ov follicular glands 

 of the mouth, the tonsils, and, at the top of the pharynx, the pharyngeal 

 tonsils, structures discovered some years ago by Koelliker. 



The lingual follicles (fig. 449) occur in man sometimes scattered, some- 

 times crowded, upon the posterior portion of the dorsum of the tongue, 

 from the cirjcumvallate papillae down to the root of the epiglottis, and 

 from one tonsil across to the other. They consist of a depression of 

 a greater or less depth (3 '5 mm. and upwards), implicating the whole of 

 the mucous membrane, so that, beside flattened epithelium, accessory 

 papillae may also exist within the reduplicated portion. 



Each crypt or depression is enveloped in a thick stratum of reticular 

 connective substance, entangled in which innumerable lymphoid cells are 

 to be found. This stratum extends to immediately beneath the epithelial 

 tunic. In it, and distinguished by their looser and wide meshed frame- 



