440 MANUAL OF HISTOLOGY. 



the contained blood, while the periodical activity of the muscular elements 

 presided over by their nervous systems leads to the expulsion of the 

 fluid contents of the spleen towards the point of least resistance, namely, 

 the veins and lymphatic vessels, and so to a decrease of volume in the 

 organ. 



In support of the theory that the spleen is a species of accessory or 

 modified lymphatic gland, providing for the reproduction of colourless 

 blood-cells, we have, in the first place, the parallelism which exists in the 

 changes taking place in both these organs in certain diseases, then the 

 greater richness of the blood of the splenic vein in white elements ( 70), 

 and, finally, the analogous structure of the spleen and lymph nodes. This 

 latter point, namely, that of similarity of structure between the spleen and 

 lymphatic glands, is most easily demonstrable in the lizard and snake, in 

 which a definitely enclosed stream of blood is seen to flow between certain 

 follicular masses (W. Muller}. 



Gray's view is that the spleen serves the purpose of a reservoir for a 

 certain quantity of the blood. Schiff, on the other hand, regards the 

 organ as an auxiliary to the digestive apparatus, yielding to the pancreas 

 its power of digesting albuminous matters. 



The origin of the spleen, as far as at present known, is from a special 

 aggregation of cells belonging to the middle germinal plate, and quite 

 unconnected with the digestive organs. These cells are subsequently 

 metamorphosed into the various tissues of which the organ is composed. 

 The first rudiments*are remarked at the end of the second month. Accord- 

 ing to Remak, the appearance of the Malpighian corpuscles is very early, 

 but Koelliker believes, on the contrary, that it is only towards the end of 

 intra-uterine life that they are formed. 



Other conclusions, however, on these points have recently been arrived 

 at by Peremeschko. According to him the spleen is developed as an off- 

 shoot of the pancreas. The envelope and trabeculse, as well as the fine 

 reticulated tissue of the pulp, is first formed ; then the lymphoid cells, 

 with numerous red blood corpuscles, make their appearance, at first in 

 but small number ; then, from the rapid increase in number of the lym- 

 phoid elements, numerous collections of the latter are formed in the 

 sheaths of the arteries, giving rise, at a very early embryonic period, to 

 the Malpighian corpuscles. 



The numerous morbid changes in structure taking place in this organ 

 require more careful attention than has up to the present been accorded 

 to them, owing to insufficient acquaintance with the minute structure of 

 the part. Among these a great increase of volume in the organ, asso- 

 ciated with an excess of white corpuscles or leucaemia, has awakened 

 much interest 



236. 



Pending a more satisfactory classification, impossible in the present 

 state of histology, we shall for the present associate with the lymphoid 

 parts a series of other organs whose functions are still a problem, and as 

 to whose structure much doubt still exists on many points. These are 

 the thyroid gland, the supra-renal body, and the pituitary body. For the 

 present we may retain for them the old name of blood-vascular glands. In 

 many cases they have already reached or passed their full development, 

 as met with in the adult body, and are engaged in retrogression or pro- 

 cesses of decay. 



