430 MANUAL OF HISTOLOGY. 



of follicles. When, however, these take a course through a swollen 

 point, or even excentrically, or through a Malpighian corpuscle, the trans- 

 formation usually goes farther, leading to the formation of a tissue nearly 

 allied to lymphoid tissue. Whilst in the lower degree of lymphoid 

 infiltration the sheath alone is affected, and not the proper adventitia, in 

 the more advanced stages of the same the latter is drawn more and more 

 within the circle of lymphoid metamorphosis. 



Turning now to the follicle, we find the sustentacular tissue framework 

 denser and more resistent peripherally, while within it possesses wider 

 meshes, and is more delicate. At times the internal is marked off from 

 the more cortical portion by a circular line, as in the rabbit, Guinea pig, 

 and marmot. This arrangement, however, calls for closer investigation. 



Here also, as in the lymph nodes, we may distinguish in some of the 

 expanded nodal points distinct nuclei. The external demarcation of a 

 Malpighian follicle is never produced by a homogeneous membrane 

 enclosing it, but always by reticular connective-tissue, even at those 

 points where by its denser texture its surface is sharply defined against 

 the adjacent structures. In other cases the follicle is continuous, as to 

 its delicate framework, with the surrounding tissue of the pulp, without 

 any sharp line of limitation existing between them. 



Entangled within the meshes of all these different portions, there 

 appear, beside free (?) nuclei (Muller), a host of ordinary lymph cells, pos- 

 sessing as a rule but a single nucleus. Some of them, however, are 

 multinuelear when very large. Beside these there occur, although in no 

 great number, elements formed of colourless granular matter, or again 

 containing molecules of a deep yellow or brown pigment. 



As regards the vessels of those portions which have become infiltrated, 

 and converted into follicles, there are also capillaries to be considered, 

 besides those arterial twigs already referred to. Veins, on the other 

 hand, are entirely absent. In parts but slightly infiltrated, is to be 

 found a slightly developed long-meshed capillary network, whereas those 

 portions greatly swelled exhibit, as a rule, a far more highly developed 

 mesh work of capillaries derived from a special and rather variable arterial 

 twig. This latter either springs from the artery of the follicle itself, or 

 approaches the Malpighian corpuscle from without. The capillary net- 

 work itself varies also ; in the first place, in different follicles of the 

 same organ, and in the second place, in different animals. It is some- 

 times met with presenting a more or less regular radiating arrangement of 

 its capillaries with arched anastomoses, the tubes having a diameter of 

 0-0029-0-0081 mm. But far more frequently the disposal of these 

 minute vessels is irregular both as to anastomosis, division, and diameter. 



Observing the texture of the capillaries more closely, we recognise 

 beside those presenting the ordinary appearance, with an adventitia, such 

 as are seen, for instance, in reticular connective substance ( 202), others 

 whose walls are exceedingly delicate wanting the double contour, but 

 which may on the other hand exhibit great richness in nuclei. In speak- 

 ing of the pulp we shall refer again to this point, which is of great im- 

 portance as regards the arrangements for the circulation in the spleen. 



In man the nature of the lymphoid infiltration, and the mode of for- 

 mation of follicles, is similar to that just described, although the trans- 

 formed arterial sheaths, and their local thickenings, may display con- 

 siderable variety. We must not forget, however, that we are obliged to 

 undertake our researches into the nature of the human spleen under 



