THE SPLEEN. 153 



parable. At first the sheaths are as thick as the fibrous coat itself 

 and they retain this thickness so long as they surround the 

 principal branches of the vessels. The finer ramifications of the 

 latter and even those small branches which are given off from 

 the large ones, have finer and finer sheaths, until at last, 

 when the vessels are quite minute, they become lost as thin 

 membranes in the pulp. The thickness of any sheath is 

 always less than that of the wall of the artery to which it 

 belongs and greater than that of the vein, but after division 

 the sheaths become relatively stronger. It was remarked above, 

 that a number of the trabecule are inserted into the vascular 

 sheaths and they therefore take a share, together with the 

 vessels which they inclose, in the formation of the dense net- 

 work in the interior of the spleen. In Mammalia, as in the Horse, 

 Ass, Ox, Pig, Sheep, &c, the sheaths present different relations, 

 inasmuch as the smaller veins have none at all, and the larger 

 possess them only on the side on which the arteries and nerves 

 lie. Only the two principal venous trunks near the hilus have 

 perfect sheaths, whilst the arteries, from the main trunks to the 

 finest ramifications, all possess them. The structure of the 

 sheaths is precisely that of the trabecules, but muscles are not 

 always found in the former when they are contained in the 

 latter — e. g. in the Ox — while in the Pig they are also very 

 distinct in the sheaths. 



The splenic artery, immediately it enters the organ, and all 

 its principal branches, divide and spread out into a great num- 

 ber of ramifications, the larger of which proceed towards the an- 

 terior margin of the organ, the smaller towards the posterior, 

 forming no anastomoses with those of other principal branches. 

 When they have diminished to the diameter of \ — ^'", they 

 separate from the veins, which till then had run in the same 

 sheath with them, and become connected by branches of 

 001 — 0'02'", with the Malpighian corpuscles in the manner 

 which has been described above; perhaps, also, sending fine 

 branches into their interior (see § 167). Then, often closely 

 applied to the surface of the corpuscles, but, so far as I 

 can observe, not passing through them, as Joh. Muller for- 

 merly supposed, they enter the red pulp and immediately break 

 up into elegant bundles of minute arteries, the so-called 

 penicilli (fig. 232), which finally subdivide into true capillaries 



