426 MANUAL OF HISTOLOGY. 



quent liquefaction of the central portion gives rise eventually to the 

 formation of the central cavities. From fig. 416, 1, representing the 

 gland of a foetal pig two inches long, in course of development, we may 

 obtain some idea of the process, and understand better the structure of 

 the gland at the period of maturity. 



The retrograde development of the gland takes place with decrease of 

 volume by the formation, as has been already remarked, of fat cells at 

 the expense of the tissue, by which we are reminded of a similar meta- 

 morphosis in the lymph nodes ( 226). That fatty degeneration of the 

 gland cells also occurs, has been asserted, as we have already said, by Ecker. 

 The time at which the retrograde process begins appears to vary; it 

 lies between the eighth and twenty -fifth years. 



229. 



We have still to consider in conclusion one other organ belonging to 

 the lymphoid series, namely, the spleen. 



Owing to the great difficulties attendant on the study of this organ, it 

 remained until a comparatively recent date the subject of but brief and 

 unsatisfactory research. But, lately, through the labours, especially, of 

 Gray, Billroth, Schweigger-Seidel, but more than all of W. Mutter, we 

 have been made acquainted with the leading peculiarities of its structure. 

 In the latter it resembles a lymph node, even more strongly than the 

 thymus. In fact, the spleen may be regarded, as I myself expressed it 

 many years ago, after careful consideration of the subject, as a lymph 

 gland in which the system of lymphatic passages is replaced by the blood- 

 vessels ; it might be named, perhaps, with propriety a blood lymph gland. 



The organ presents, in accordance with this view beside, a. fibrous enve- 

 lope with a system of trabeculce or septa, and a sheath-like formation of 

 connective-tissue around the vessels, a soft glandular parenchyma. The 

 latter is of two kinds ; it presents itself, in the first place, in the form of 

 lymphoid follicles, and in the next as a brownish red friable mass, known 

 as the pulp of the spleen. The first of these correspond to the elements 

 of the same name found in the lymph nodes ; the latter is more or less a 

 modified species of the medullary substance. 



Beneath the serous covering, which may be isolated from the organ in 

 the ruminant body, the fibrous envelope or capsule of the spleen appears. 

 In man, on the contrary, this tunic is closely adherent to the investing 

 peritoneum. It is seen, under the microscope, to be made up of a dense 

 interlacement of connective-tissue fibrillse, with a preponderance of fine 

 elastic fibres, and contains also unstriped muscular elements. The latter 

 are present in large numbers in many of the mammalia, as, for instance, 

 in the sheep, dog, pig, horse, and hedgehog, especially in the deeper por- 

 tions of the envelope. In other animals of this class they do not make 

 their appearance in such quantities, as, for example, in the ox ; while in 

 man the contractile fibre cells are present in but small proportion. 



The capsule which invests the whole spleen is folded in at the point of 

 entry of the vessels and nerves, the so-called hilus, and is continued 

 further inwards in the form of sheaths to the various vessels. It accom- 

 panies the ramifications of the latter (more strongly developed and massive 

 around the arteries than the veins) down to their finest twigs. It 

 exhibits, however, considerable variety in the various species of animals, 

 a point to which we shall be obliged to refer again further on. 



Besides the sheaths of the vessels, and continuous with them, we meet 



