87 



LESSON XX. 



STRUCTURE OF THE LARGER BLOOD-VESSELS. 



1. SECTIONS of a medium-sized peripheral artery and vein. In this pre- 

 paration the limits of the vascular coats can be well seen and also the differ- 

 ences which they present in the arteries and veins respectively. The sections 

 may either be stained with hsematoxylin and mounted in Canada balsam, or 

 they may be stained in dilute mageuta and mounted in glycerine and water. 



2. Mount in Canada balsam a thin slice cut from the inner surface of an 

 artery which, after having been cut open longitudinally and washed with 

 distilled water, has been treated with nitrate of silver solution and exposed to 

 the light in spirit. This preparation will show the outlines of the epithelioid 

 cells which line the vessel. 



3. A piece of an artery which has been macerated for two or three days 

 in 30 per cent, alcohol (1 part rectified spirit to two parts water) is ta be 

 teased so as to isolate some of the muscular cells of the middle coat and 

 portions of the elastic layers (networks and fenestrated membranes) of the 

 inner and middle coats. The tissue may be stained cautiously with dilute 

 logwood solution, and glycerine afterwards added. The muscular cells are 

 recognisable by their irregular outline and long rod-shaped nucleus. Sketch 

 one or two and also a piece of fenestrated membrane. 



4. Transverse section of aorta. Notice the differences in structure 

 between this and the section of the smaller artery. 



5. Transverse section of vena cava inferior. Notice the comparatively 

 thin layer of circular muscle, and outside this the thick layer of longitudinal 

 muscular bundles. 



Make sketches from 1, 4, and 5, under a low power, from 2 and 3 under 

 a high power. 



An artery is usually described as being composed of three coats, 

 an inner or elastic, a middle or muscular, and an external or areolar 

 (fig. 109, b, c, cZ). It would, however, be more correct to describe the 

 wall of an artery as being composed of muscular and elastic tissue 

 lined internally by a pavement-epithelium and strengthened externally 

 by a layer of connective tissue. For the present, however, we may 

 adhere to the generally received mode of description. The inner coat 

 of an artery is composed of two principal layers. The inner one is a 

 thin layer of pavement -epithelium (often spoken of as the endothelium), 

 the cells of which are somewhat elongated in the direction of the axis 

 of the vessel (fig. 110), and form a smooth lining to the tube. After 

 death they become easily detached. Next to this comes an elastic 



