104 



STUDENTS HISTOLOGY 



quite continuous, as minute openings are to be seen at irreg- 

 ular intervals.* 



The AORTA has intima, media, and adventitia like the other 

 arteries. The preponderance of the elastic tissue over the muscular, 

 which is characteristic of large arteries, reaches its fullest develop- 

 ment in the aorta. The intima is thick, and is not well marked off 

 from the media. The great amount of elastic tissue in large 

 arteries is connected with their function of converting the pulsating 

 blood -current into a steady stream. The muscular fibers serve to 

 control the calibers of arteries and the amount of blood flowing to 

 any part. 



FIG. 74. ISOLATED BLOOD-CAPILLARIES. 



A. Plexus from a pulmonary alveolus, stained- with silver (X 350). 



B. Capillary from omentum, stained with silver and haematoxylyi (X 700). 



In A the cells are outlined by the silver ; while in B the nuclei in addition are brought out 

 by the hsematoxylin. 



The walls of veins are much thinner than those of arteries. 

 The intima presents an endothelial lining, but the line of demarca- 

 tion between this coat and the media is often indistinct. The 

 media contains muscular tissue but not much elastic tissue; and 

 the Adventitia, usually the most prominent of the three coats, is 

 composed largely of fibrous connective tissue. 



The valves of the veins are reduplications of the intima, having 

 a semilunar form, and with the fibrous tissue well developed. 



*It is probable that what appear to be openings between endothelial cells are, in fact, occu- 

 pied by cement substance. In conditions of congestion and inflammation they become actual 

 holes, and facilitate the migration of the leucocytes from the vessels by their amo?boid move- 

 ment, and permit, also, the diapedesis or escape of red corpuscles without rupture of blood, 

 vessels. (Klein.) 



