376 



MANUAL OF HISTOLOGY. 



indeed frequently, to a considerable extent, in regard to the various parts 

 of the same. 



As to their relation to the blood-vessels one thing is certain, that a 



Fig. 375. Surface of the vermiform ap- 

 pendix of the rabbit, a, a depression ; 

 6, outlets of the follicles of lAeberkiihn ; 

 c, lymphatic network; d, descending 

 canals. 



Fig. 376. Papilla from the colon of the 

 rabbit, a, arterial; b, venous twig; c, 

 capillary network ; d, descending vein of 

 the papilla; e, lymphatic vessel; /, lymph 

 canals of the papilla; g, blind termina- 

 tion of the latter. 



transition of one into the other takes place nowhere, neither directly nor 



with the interposition of fine tubules. 



In many instances we may 

 see lymph canals surrounded 

 externally by capillaries (fig. 

 372, 376). In other cases 

 tubes of both kinds run, with 

 greater or less regularity, side 

 by side (fig. 377). 



Finally, a stream of lymph 

 may be taken up by the ad- 

 ventitia of a blood-vessel, en- 

 veloping the latter as with 

 a sheath (fig. 376, <?). Thus 

 we see that the arrangement 



f ,1 , , , 



01 the Structures in question 



is V ery various. 



A few moments must now be devoted to the consideration of that 

 ensheathing of blood-vessels in lymph streams which has just, been men- 

 tioned. 



It has been the custom to speak of this as of frequent occurrence 

 among the lower orders of vertebrates, such as reptiles. It has been 

 denied, however, that it takes place at all in the frog by Langer, who has 

 given the matter his.closest attention. In the higher animals and in man 

 it may appear, without, however, being anything but an accidental 

 occurrence, except, perhaps, in particular portions of the body. 



We have recently learned from His that the blood-vessels of the 

 nervous centres, the brain and spinal cord, are loosely enveloped in a 

 sheath of streaked connective-tissue in a vast number of cases. This 

 arrangement of parts is seen among arteries, veins, and capillaries alike, 



Fig. 377. From the testicle of the calf, a, seminal tubules 

 in side view; 6, more obliquely seen; c, blood vessels ; d, 



lymphatic canals. 



