ORGANS OF THE BODY. 



429 



from four to six branches. Down to twigs cf about 0*2030 mm. they 

 are contained in a common sheath, possessing a thickness of about 0-2256 

 mm. at its commencement. This investing formation then becomes 

 gradually finer and finer, until reduced to a thickness of 0-1128 mm., 

 enveloping in this state arteries having a diameter of 0"2256 mm., and 

 veins of 0-4512 mm. 



The arterial twigs with their sheaths then separate by degrees from the 

 accompanying veins, and ramify independently. But about the venous 

 tube the simple sheath extends somewhat farther still, becoming even- 

 tually split up into fibres, continuous with the trabeculae of the organ 

 (W. Muller). 



These sheaths exhibit the same minute structure, farther, as the trabeculse. 



At those points, however, at which the arterial separates from the 

 venous twig, the structure of the tunic of the former changes its nature. 

 Its fibrous tissue is transformed into reticular lymphoid connective sub- 

 stance, together with which a decrease in its amount goes hand in hand. 

 The advancing metamorphosis also, commencing externally, attacks even- 

 tually the proper tunic of the artery. Progressing still, this trans- 

 formation, this construction of " lymph sheaths " gradually leads to more 

 or less circumscribed swellings of different shapes, and these finally 

 to the Malpighian corpuscles of the spleen (fig. 419, a). In fact, the 

 latter, with their varied configuration sometimes roundish, in other 

 instances elongated more or less, and possessing a diameter of 0-2256- 

 0'7444, or on an average 0'3609 mm., take their origin from the infil- 

 trated sheaths of the arteries, from which they can be by no means 

 sharply defined. 



Arterial twigs of 0-1579 and 0-0993 mm. in diameter, clown to those 

 of only 0'0203, are usually seen to possess this metamorphosed sheath, 

 and may all acquire a great increase of size by its formation. 



Owing, however, to the fact that the position of the artery is by no means 

 always the same as regards 

 this infiltrated sheath, fur- 

 ther variety may be noticed. 

 The former, namely, may 

 pass either through the axis 

 of these elongated masses or 

 more laterally. In those 

 parts, also, converted into 

 follicles, we meet, at one 

 time, with an eccentric 

 course in the arterial twig, at 

 another with a more central 

 one. 



This position, further, has 

 an effect on the texture of 

 the various parts of the 

 sheath. In the lower degrees 

 of transformation we usually 

 meet with an ordinary 

 loosely woven connective- 

 tissue with lymph cells in 

 its interstices. The same is 

 the case with the sheaths of the arterial twigs passing along the borders 



Fig. 419. Section of the spleen of a rabbit a, Malpighian 

 corpuscle; 6, su.stcntacular niatter of the pulp, with the 

 interspaces filled with venous blood. 



