EPITHELIAL AND ENDOTHELIAL TISSUE. 333 



THE VASCULAR SYSTEM. 



The vascular system is formed from the middle germinal 

 layer, the mesoblast, and especially of the connective tissue, 

 which alone carries blood-vessels. The different portions of this 

 system are the heart, the arteries, the capillaries, and the veins. 

 In the heart the muscular apparatus is highly developed, as this 

 is the principal motor for the blood current. Arteries always have 

 a muscle coat, which is comparatively more developed the smaller 

 the caliber of the vessel, evidently to admit of an independent 

 activity in the motion and distribution of the blood. The veins 

 have a thinner coat of muscular tissue than the arteries, while 

 the capillaries have no muscular investment whatever. The 

 layer constantly present in the heart, as well as in all blood- 

 vessels, is the most internal viz. : the endothelial. This, as 

 previously mentioned, is the representative of the investment of 

 living matter of the vacuoles, which are visible at times in single 

 plastids. The endothelial layer, in the earliest stages of forma- 

 tion of blood-vessels, is a continuous investment around the 

 vacuoles, and in a later stage of development divides to form 

 single plastids, which, although separated from each other by a 

 narrow rim of cement-substance, still remain interconnected 

 throughout life by means of delicate filaments (" thorns "). The 

 cement-substance, under certain conditions, becomes permeable 

 to both colored and colorless blood-corpuscles. The nourishing 

 material the plasma of the blood must necessarily penetrate 

 the endothelial layer before it can reach the neighboring tissue, 

 and the number of -blood- vessels is always proportionate to the 

 activity of the tissue in which they lie. Muscles f. i., glands, 

 the gray substance of the nerve-centers are abundantly sup- 

 plied with blood-vessels ; while the comparatively inert cartilage, 

 on the contrary, has a very limited supply. Blood-vessels are 

 found in the greatest numbers in the organs in which the oxyda- 

 tion of the blood is accomplished i. e., the lungs. What influ- 

 ence the bioplasson of the endothelia has upon the exchange of 

 liquids and gases we are unable to say ; this much is certain, 

 however, that all endothelia are bioplasson formations, manifest- 

 ing vitality in a high degree, and not " elastic plates," as some 

 histologists claim. The striking changes that occur in endo- 

 thelia during inflammation are a direct proof of their life and 

 activity. 



(1) The Heart. The muscle of the heart is composed of branch- 



