1 66 HISTOLOGY 



These factors are further considered by Thoma (Histomechanik des 

 Gefasssystems, 1893). 



The way in which main trunks develop from indifferent networks has been described 

 by Evans on the basis of extraordinarily perfect injections; thin fluid introduced into 

 the vessels of a living chick embryo is distributed throughout the vascular system 

 by the action of the heart (Anat. Rec., 1909, vol, 3, pp. 498-518). Obviously how- 

 ever if vessels are arising as mesenchymal spaces which subsequently become joined to 

 the vascular system, they would not be revealed by this method. The existence of 

 detached spaces in rabbit embryos has been denied by Bremer, after making very 

 careful graphic reconstructions of all the vessels in the anterior end of the specimens 

 studied. He finds that a network consisting largely of solid strands precedes the 

 network of open tubes (Amer. Journ. Anat., 1912, vol. 13, pp. 111-128). Schafer, 

 however, describes the formation of vessels by the vacuolization of connective 

 tissue cells, which then become connected with processes from pre-existing capillaries, 

 and so added to the endothelium. He states that "a more or less extensive capillary 

 network is often formed long before the connection with the rest of the vascular 

 system is established" (Text-book of Micr. Anat., 1912). His observations were 

 made upon subcutaneous tissue of the new-born rat. Similar appearances in the 

 subcutaneous tissue of human embryos may be interpreted quite differently, and 

 before it can be accepted that the cells containing red corpuscles are detached from 

 the vascular system, careful reconstructions are required. 



The formation of anomalous vessels readily takes place by the persistence and 

 enlargement of channels usually unfavorable. This is discussed by S. R. Williams in 

 explaining the condition observed in an adult salamander, in which one of the long and 

 slender lungs received its artery at the anterior end and the other at the posterior 

 end (Anat. Rec., 1909, vol. 3, pp. 409-414). Innumerable forms of human vascular 

 anomalies may thus be explained embryologically; some of them represent persistent 

 vessels which are normally important at a certain stage of development, and others 

 represent connections which are as abnormal in the embryo as in the adult (cf. Lewis, 

 Amer. Journ. Anat., 1909, vol. 9, pp. 33-42). 



A very characteristic form of circulation occurs in certain organs, in 

 which the endothelium of the vessel walls is closely applied to the epithe- 

 lium of the secreting tubules, or other parenchymal structure (Fig. 160). 

 The walls of the vessels are as thin as those of capillaries, but their di- 

 ameter is much greater, so that they have been described as lacunar ves- 

 sels or "sinusoids," the term sinus being generally applied to the large 

 thin-walled veins in the dura mater about the brain (Minot, Proc. Boston 

 Soc. Nat. Hist., 1900, vol. 29, p. 185-215). Apparently the close ap- 

 position of the endothelium, on all sides, to the cells of the parenchyma 

 is the most essential characteristic of these vessels and must be of con- 

 siderable physiological significance. There are few or no connective 

 tissue cells between the thin lining of the vessel and the epithelial tissue 

 which it nourishes. Capillaries, on the contrary, are imbedded in con- 

 nective tissue, even though occasionally they approach close to an epithe- 

 lium, sometimes appearing to enter it. In the lungs the capillaries are 

 compressed between epithelial plates, but they do not resemble the ves- 

 sels shown in Fig. 160. 



