362 



MANUAL OF HISTOLOGY. 



vesicles of the organ. 

 similar plan. 



The formation of the lungs is carried out on a 



17. The Vessels. 

 201. 



Of a special vascular tissue, or tissue peculiar to vessels, we can only 

 speak in a very limited sense. The most internal layer alone consists 

 everywhere of a series of flattened cells of a peculiar kind, cemented 

 together at their edges, and resembling very closely epithelium. The 

 walls of the finest and most simple tubes are composed solely of these 

 cells. All the remaining coats, on the other hand, which strengthen the 

 walls by being laid down around them (and they are seen very early) are 

 formed of muscular and elastic tissue, of structures, therefore, to which 

 we have already given our consideration. But in that the fine tubes with 

 their simple texture are continuous K through the most gradual transitions 

 with those of wider gauge and more complex structure, a general glance 

 at the blood-vessels and lymphatics will be found useful. 



It is well known that the canals 

 of the vascular system are classified 

 into those which convey the stream of 

 blood from the heart, called arteries, 

 those which collect and bring back 

 the same known as veins, and those 

 which are interposed between these 

 two, forming a system of fine hair- 

 like tubes, to which the name of 

 capillaries has been applied. The 

 latter, compared with the merely con- 

 ducting veins and arteries, constitute 

 the most important part, physiologi- 

 cally, of the whole, in that through 

 their delicate walls the interchange 

 of matter between the blood and 

 organic fluids, as well as secretion, 

 takes place. 



The capillary vessels present for 

 our consideration, as a rule, a wall 

 quite distinct from the neighbour- 

 ing structures. For those so con- 

 stituted we would retain the name 

 of capillaries. In other and rarer 

 instances, this tube containing the 

 blood is fused with the adjacent 

 tissues, the fluid, as it were, flowing 

 through grooved passages, in which 

 case we have the capillary canal. 

 Finally, recent observations seem to 

 teach that in the pulp of the spleen 

 the finest streams of blood actually 

 flow through membraneless inter- 

 stices. These latter are known as 

 capillary lacunae. 

 Capillaries of the smallest calibre, which do not, however, occur in all 



Fig. 355. Fine blood-vessels from the pia mater 

 of the human brain. A, a small branch, 

 which divides above into two delicate capil- 

 laries, o, 6, and which consists below (d), of two 

 tunics; , a similar tube, with branches; C, a 

 vessel of greater calibre, with a double mem- 

 brane, the internal (a) snowing longitudinally 

 arranged, and the external (6), as well as 

 intermediate, transverse nuclei. 



