NINTH LECTURE. 



THE BLOOD-VESSELS. 



ONE cannot really speak of a vascular tissue. Only the 

 innermost layer consists of a simple layer of closely cemented 

 endothelial cells. This is the original stratum ; it forms the 

 simplest, finest vascular tube. 



All the remaining layers, on the contrary, which by their 

 further aggregation reinforce the walls of the vessel — and 

 they commence very soon — belong to tissues which we have 

 already discussed ; they consist of connective tissue and elastic 

 substances, as well as layers of smooth muscles. 



The blood is conducted from the heart, as is known, 

 by immensely ramified systems of arteries. Its return is 

 consigned to the not less ramified veins. Between them 

 is intercalated, but without any sharp demarcation, the 

 district of the capillaries. They maintain the nutrition 

 of the organs and tissues, as well as the secretion of the 

 glands. 



The finest capillaries — they do not by any means occur in 

 all parts of the body, however — have a calibre which just 

 suffices to permit the passage of the blood cells, one after the 

 other, though often with a certain lateral compression. Their 

 lumen may, therefore, be assumed to be for man 0.0045 to 

 0.0068 mm. In other parts of the body, however, the finest 

 capillaries present double this diameter. 



Without being treated with a suitable reagent, their struc- 

 ture appears extraordinarily simple. A hyaline, structure- 

 less, extensible and elastic membrane contains embedded, 

 from place to place, rounded or elongated oval nuclei of 

 0.0056 to 0.0074 mm., with nucleoli. In the finest capil- 

 laries the nuclei lie in the simplest manner behind each 



