470 MANUAL OF HISTOLOGY. 



work, and consequently lighter shade, a number of small lymphoid 

 follicles may be observed, measuring in diameter from 0'28 to 0'56 mm. 

 These are sometimes sharply denned, sometimes less distinctly so. Other 

 of these crypts, however, are quite destitute of these follicles. Most 

 usually we find these lingual crypts encased in a strong fibrous capsule. 

 This, however, is also often absent in less distinctly defined . examples. 

 Among and beneath these lingual follicles are 

 generally scattered a great number of racemose 

 glands, whose ducts open partly in the immediate 

 neighbourhood of the crypt (but on the surface of 

 the mucous membrane), and partly within its 

 cavity. In many mammals these lingual follicles 

 are entirely absent, as in the tongue of the 

 rabbit, sheep, and dog. In others they are formed 

 upon the same plan as in man, for instance, 

 in the horse, pig, and ox. 



Fig. 449. Plan of a lingual rr , , , * ' , 7 , , . , , 



follicle, a, hollow redupH- Ihe blood-vessels and lymphatics are ol the same 

 suerith f itspapiii* C - 6 S iymI description as those in the tonsil, to which we 

 phoid portion of the 'walls refer the reader for minutiae. 



The tonsils or amygdala*, the largest lymphoid 



organs of the mouth, are to be found in man and most of the mammalia, 

 presenting, however, considerable variety of structure in the latter. In 

 some of these, moreover, as in the Guinea-pig, the rat, and the mouse, 

 they are entirely absent. The form of the organ, as it appears in the 

 rabbit and hare, is from its simplicity very instructive. Here we find 

 a simple depression surrounded by a thick stratum of lymphoid structures 

 containing small lymph-follicles. The boundary of the organ, externally, 

 is a fibrous capsule. Numerous minute racemose mucous glands lying 

 adjacent, send off their ducts, partly external to the depression, and partly 

 through the lymphoid mass, to empty themselves into the latter. In this 

 case, therefore, these organs show all the characters of a lingual follicle. 



As a rule, however, the tonsils present^ far more complicated structure. 

 They are generally made up of groups of such bodies as those we have 

 just described as representing the tonsils in the hare. These are collected 

 together in greater or less number, their follicular ducts opening either 

 singly on the surface or converging like the corresponding portions of a 

 racemose gland to form passages of greater magnitude. These latter may 

 then discharge their contents, either independently of one another, or, 

 pursuing the same system of confluence, may eventually, as in the tonsil 

 of the ox, give origin to one large excretory duct. Between these two 

 extremes many intermediate forms are met with. 



Each pit is enveloped in a thick lymphoid layer external to the 

 flattened epithelial lining and mucous membrane papillaB often present. 

 This layer, enveloped in dense fibrous tissue, extends as far as the 

 epithelium or its immediate vicinity. As a rule, but not invariably, it 

 contains within a loose tissue a number of follicles. 



The latter present, both as to number and distinctness of demarcation 

 against tne denser interstitial tissue around, considerable variety. Their 

 diameter, in most mammals, is on an average about 0'28 or 0*51 mm. 

 In the dog they may even attain greater magnitude, reaching 0'9-1*4 mm. 

 The large tonsils of the pig, further, are unusually rich in follicles. 



Here also are to be met with, as might be expected, numerous racemose 

 mucous glands, playing an important part in the construction of the 



