MOVEMENT OF THE BLOOD IN THE CAPILLARIES. 345 



regains its previous velocity and force, and flows into exactly the same pas- 

 sages as before. It is still doubtful whether the capillaries possess any con- 

 tractile power of their own, or merely vary in size owing to the elasticity of 

 their walls, with the quantity of blood they contain. 1 The capillaries in 

 certain regions, 2 as the central parts of the nervous system and the mesen- 

 tery, appear to be surrounded by a sheath between which and their own 

 proper coat, lymph-corpuscles have been observed ; these spaces have con- 

 sequently been regarded as extensions of the lymphatic system. Frey 

 remarks, however, that it is not every adventitial tissue of a bloodvessel 

 containing lymphoid cells that must be regarded as a lymph sheath. A 

 circumstance that may frequently give rise to the deceptive appearance of 

 such a sheath, is that a bloodvessel is often bounded on each side by lym- 

 phatic canals, and this is most commonly seen in uniujected preparations. 



261. The opinion was long entertained, that there are vessels adapted to 

 supply the white or colorless tissues; carrying from the arteries the "liquor 

 sanguiuis," and leaving the corpuscles behind, through inability to receive 

 them. Very well-marked examples of such non-vascular tissues are found 

 in the cornea and hyaloid humor of the eye, and in the mucous tissue con- 

 stituting the greater part of the umbilical cord. A considerable develop- 

 ment has lately been given to this view by the investigations of Virchow 3 

 and others ; by whom it has been attempted to be shown that in the various 

 structures included under the term connective tissues, the corpuscles, which 

 are almost constantly found disseminated through them, are to be regarded, 

 like the lacunae of bone, as centres and storehouses of nutriment, which is 

 again distributed by their caudate prolongations (frequently assuming the 

 for,m of elastic tissue) to the most distant parts. The idea that Nutrition 

 can only be carried on by means of Capillary vessels is, however, entirely 

 gratuitous ; for there is no essential difference between the nutrition of the 

 non-vascular tissues, and that of the islets in the midst of the network of 

 capillary vessels which traverse the most vascular. In both cases, the nu- 

 trient materials conveyed by the blood are absorbed by the cells or other 

 elementary parts of the tissue immediately adjoining the vessels, and are 

 imparted by them to others which are further removed ; and the only dif- 

 ference lies in the amount of the portion of tissue which has to be thus 

 traversed, so that we are only required to extend our ideas, from the largest 

 of the islets which we find in the vascular tissues, to the still more isolated 

 structures of which the non-vascular tissues are composed. The disposition 

 of the Capillaries, both as to the degree of minuteness and the plan of the 

 reticulation which they form, varies so greatly in the different vascular 

 tissues, that it is possible to state with tolerable certainty the nature of the 

 part from which any specimen has been detached, whether a portion of 

 Skin (Fig. 152), Mucous membrane (Fig. 153), Serous membrane. Muscles 

 (Fig. 154), Nerve Fat (Fig. 155), Areolar tissue, Glands, etc. The degree 

 of minuteness is obviously in accordance with the copiousness of the supply 

 of blood which is required for the purposes of its circulation through the 

 part ; thus the plexus is closest, where some change is to be effected in the 

 blood itself, as in the absorbent, respiratory, and secreting organs; whilst it 

 is widest in those parts which receive the blood solely for their own nutri- 

 tion, the nervous centres and muscles having a more minute reticulation 



1 See Eberth, op. cit., and Strieker, Wien. Sitz.-berichte, Bd. li and lii, for argu- 

 ments in favor of the former view, which are vigorously contested by Beale, op. cit., 

 and Vulpian, Lemons sur 1'Appwreil Vaso-moteur, 1S75, p. 73 & 



2 Strieker, in Moleschott's Untersuch., Bd. x, Het't 2, p. 168. 



3 See Cellular Pathology, passim. 



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