IT 4 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



neck are so placed as to become functional when an 

 animal lowers its head, as in the act of grazing. 



SUMMARY OF ARTERIES AND VEINS. 



I. Tunica intima. 



1. Endothelium, simple squamous epithelial cells. 



2. Subendothelial layer. 



*(a) White connective-tissue fibers. 

 (b) Connective-tissue cells. 

 t(c) Elastic connective-tissue fibers. 



ts. Henle's fenestrated membrane (elastic internal limiting mem- 

 brane). Interlacing basketwork of elastic fibers. 

 II. Tunica media. 



ft. Smooth muscle, circular. 



f2. Elastic plates and fibers, longitudinal and circular. 



3. Nerves. 



4. Blood capillaries, difficult to demonstrate. 

 *S- White connective fibers. 



6. Connective-tissue cells. 



7. Muscle fibers longitudinal, rare. 

 III. Tunica adventitia. 



*i. White connective-tissue fibers, longitudinal and oblique. 



2. Connective-tissue cells. 



f3. Elastic connective-tissue fibers, longitudinal (external limit- 

 ing membrane) . 



4. Nerves. 



5. Vasa vasorum (blood-vessels). 



6. Lymphatic vessels and nodes. 

 *7. Smooth muscle fibers. 



It should be remembered that structural difference 

 in large and small arteries is in keeping with their 

 function. In small arteries or arterioles, the involun- 

 tary muscle is conspicuous, as it is the contraction of 

 this muscle that regulates the blood supply to an 

 organ. In large arteries, as the aorta, the muscle is 



* This tissue predominates in veins. 

 fThis tissue predominates in arteries. 



