122 MICROSCOPIC AX A TO MY OF THE ORGANS. 



I. BLOOD VASCULAR SYSTEM. 



This whole system is lined with endothelium, which forms 

 a complicated epithelial or endothelial tube. The heart is 

 much thickened outside this tube by muscular development, 

 while the capillaries consist of the endothelial tube alone. The 

 thicker vessels have the so-called accessory coats, which are 

 divided into intima, media, and adventitia. 



(a) Capillaries. 



These are very small vessels with a diameter of between 

 7 and 15 /w, situated between the venous and arterial systems. 

 The wall consists of a single layer of flat endothelium, the cells 

 of which are joined together by a small quantity of cement 

 substance, which is easily demonstrated by means of silver 

 nitrate. The outlines of these cells are shown in Fig. 93, and 

 are seen to be irregular and jagged. They are arranged usually 

 with their long axes parallel to that of the vessel. The nuclei 

 are arranged similarly and show a small collection of proto- 

 plasm in their immediate neighborhood. Stigmata and stomata 

 (small openings between the cells) are seen frequently. The 

 thinness of the walls allows diffusion and osmosis to go on 

 freely, which is of great importance in the body metabolism. 



The capillaries anastomose freely with one another, forming 

 a network which is easily seen in the mesentery In muscle 

 the meshes of this network are long and narrow. In the liver 

 they are much smaller. 



The vessels which pass over from the capillaries to the 

 arteries, on the one hand, and to the veins on the other hand, 

 are known as precapillary arteries and veins. 



The new formation of capillaries is seen best in the greater 

 omentum of young animals (Fig. 86). From already devel- 

 oped capillaries protoplasmic sprouts branch off and extend 

 into the surrounding tissue. These sprouts are the result of 

 karyokinetic division of the endothelial cells. They begin to 

 become hollow and to form a blind canal, which meets another 

 similar sprout and joins with it, the lumen becoming con- 

 tinuous in the two. Other authors describe special vasoforma- 



