102 . STUDENTS HISTOLOGY 



THE CIRCULATORY SYSTEM 

 THE HEART 



The muscle of the heart has already been described. The 

 muscle-cells are supported by a small amount of connective tissue, 

 in which run the blood- and lymphatic vessels and nerve -fibers. 

 Both medullated and non-medullated nerve-fibers are supplied to 

 the heart, and minute ganglia also occur, especially in the auriculo- 

 ventricular and the inter -ventricular furrows. 



The PERICARDIUM is one of the great serous membranes. Its 

 surface is covered by a single layer of flat endothelial cells, beneath 

 which is a stratum of fibro- elastic connective tissue. This con- 

 nective tissue is continuous with that running between the muscle- 

 fibers. Underneath the pericardium there is usually more or less 

 adipose tissue, especially along the course of the larger blood* 

 vessels. 



The ENDOCARDIUM is covered with a single layer of flat endo- 

 thelial cells, which rest upon fibro -elastic connective tissue. The 

 connective tissue is continuous with that supporting the muscle- 

 fibers, and also joins with the lining of the blood-vessels that open 

 into the heart. 



The valves of the heart are duplications of the pericardium, 

 containing abundant connective tissue. The muscle -fibers of the 

 auricle extend a short distance into the auriculo- ventricular valves. 

 A few blood-vessels may be present at the attached borders of the 

 valves. 



BLOOD- VESSELS 



Blood-vessels include arteries, arterioles, capillaries, venules, and 

 veins. They are all lined with flattened endothelial cells cemented 

 by their edges; and their walls are constructed from non- striated 

 muscular, yellow elastic, and fibrous connective tissues, in propor- 

 tions varying according to the size and function of the vessel. 

 Arteries are active, while the veins are comparatively passive 

 agents in the circulation of the blood. 



The large arteries are eminently elastic, from preponderance of 

 yellow elastic tissue; while the arterioles are eminently contractile, 

 from excess of muscular fiber. 



