112 THE ESSENTIALS OF HISTOLOGY. 



LESSON XXIII, 



SMALLER BLOOD- VESSELS. L YMPHA TIC SYSTEM. 



1. TAKE a piece of pia mater which has been stained with hasniatoxyliu, and 

 separate from it some of the small blood-vessels of which it is chiefly com- 

 posed. Mount the shreds in Farrant or dilute glycerine. The structure of 

 the small arteries can be studied in this preparation, the nuclei of the epi- 

 thelium and of the muscular coat being brought distinctly into view by the 

 stain. The veins of the pia mater possess no muscular tissue. Capillary 

 vessels which have been dragged out from the brain in removing the pia 

 mater may also be seen in this preparation. Sketch two small arteries of 

 different sizes, giving also their measurements. 



2. Mount in Canada balsam a piece of the omentum of the rabbit, stained 

 with silver nitrate. The membrane should be stretched over a cork or a ring 

 of glass or vulcanite, rinsed with distilled water, treated for five minutes 

 with 1 per cent, nitrate of silver solution, again washed and exposed to sun- 

 light in spirit. When stained brown the preparation is removed from the 

 light. Pieces may now be cut off from the membrane and mounted, as 

 directed, in Canada balsam ; they should include one or more blood-vessels. 



This preparation is intended to show the epithelium of the smaller blood- 

 vessels and accompanying lymphatics, and also the epithelium of the serous 

 membrane. Sketch a small piece showing the epithelium of the vessels. 



3. Mount in Canada balsam a piece of the central tendon of the rabbit's 

 diaphragm which has been similarly prepared (except that the pleural 

 surface has first been brushed to remove the superficial epithelium so as to 

 enable the nitrate of silver more readily to penetrate to the network of 

 lymphatic vessels underlying that surface). Observe the lymphatic plexus, 

 under a low power ; sketch a portion of the network. If the peritoneal 

 surface is fo.cussed, the epithelium which covers that surface will be seen, and 

 opposite the clefts between the radially disposed tendon-bundles stomata may 

 be looked for in this epithelium. 



4. Carefully study the circulation of the blood either in the web of the 

 frog's foot or in the mesentery or tongue of the frog or toad, or in the tail of 

 the tadpole. 



The coats of the smaller arteries and veins are much simpler in 

 structure than those of the larger vessels, but they contain at first all 

 the same elements. Thus there is a lining epithelium (endothelium) 

 and an elastic layer forming an inner coat, a middle coat of circularly 

 disposed plain muscular tissue, and a thin outer coat. The same differ- 

 ences also are found between the arteries and veins,, the walls of the 

 veins being thinner and containing far less muscular tissue (fig. 137), 



