424 MANUAL OF HISTOLOGY. 



These are the elements of the gland, the so-called granules or acini of the 

 thymus. At first sight they remind us forcibly of lymphoid follicles. 

 Under closer observation, however, important differences manifest them- 

 selves. Externally, these acini of the thymus are separated from one 

 another by deep indentations, whereas, internally, they become united, 

 as many as fifty of them together, to form, a medium-sized lobe recall- 

 ing to mind the state of things observed in the racemose glands. Then 

 and great stress must be laid upon this point the thymus element appears 

 hollow in its interior, and the cavities of the thick-walled acini of each 

 lobe unite, as in the racemose glands, to form its common passage. This 

 then joins with similar canals belonging to other lobes, until, by a repeti- 

 tion of the occurrence, the spiral central canal of each half of the organ is 

 produced. 



Even in the walls of this common duct, bulgings or attached acini, or 

 groups of the same, may be remarked, so that its thickness varies at 

 different points. 



As to the texture of the acinus, we find that the central cavity, occu- 

 pying about J-J- of the whole diameter, is bounded by a layer of soft 

 tissue. This consists of an exceedingly dense network of stellate cells of 

 reticular connective-tissue. The narrow meshes of this structure are occu- 

 pied here,. as in the lymphoid follicles, by an immense number of lymph 

 corpuscles. A very delicate membrane, richly supplied with blood-vessels, 

 covers its surface. The blood-vessels, farther, which traverse the follicular 

 tissue, are also very numerous, and possess the well-known adventitia 

 ( 202). With the exception of a few stronger twigs, these are for the 

 most part capillaries of 0'0063-0-0068 mm. in diameter. Injection of 

 these brings out their arrangement in the most instructive manner. 



From the larger vessels of the central band smaller twigs are given off 

 to the lobuli. Here they eventually form (in the calf) delicate circular 

 and arched groups of arterial and. venous branches (fig. 417, a, b) around 

 the individual acini. Springing from these the capillaries are seen inter- 

 nally (c) taking a convergent course, and forming a most exquisite net- 

 work amid the lymphoid substance. Close to the central cavity they 

 double on themselves (d) according to His. 



In the thymus of the infant, though the arrangement of the capil- 

 laries is the same, an exception is so far to be seen to this arrangement 

 of parts, that while the vein courses along at the periphery of the acinus as 

 in the calf, the artery and its system of finer tubes occupies the interior of 

 the glandular tissue near the central cavity in a manner wholly different. 



In the small meshes of the reticulum it has been asserted that numbers 

 of free nuclei may be seen (fig. 416, a) suspended in an acid, viscid, 

 albuminous fluid. The essential element is, however, beyond doubt a 

 small nucleated lymphoid cell (b) measuring 0'0074 mm. in diameter. 

 More rarely we meet with large cells of from 0*0046 to 0'0023 mm., 

 containing several nuclei, from 2 to 8. Ecker mentions further, as a 

 phenomenon of retrograde development, that a deposit of fat globules 

 takes place (d) in many cells, which, as soon as the organ is on the 

 decline, run together to form one large drop, filling the whole body of the 

 elements in question (e,f). He states, besides, that in older cells he has 

 not unfrequently observed an absence of nuclei (/). 



But there are other structures to be met with here which are by no 

 means connected with the involution of the thymus ; these are the so- 

 called concentric bodies. 



