92 THE ESSENTIALS OF HISTOLOGY 



LESSON XXI. 



SMALLER BLOOD-VESSELS. LYMPHATIC SYSTEM. 



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

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

 composed. Mount the shreds in Farrant. The structure of the small 

 arteries can be studied in this preparation, the nuclei of the epithelium and 

 of the muscular coat being brought distinctly into view by the logwood. 

 The veins, however, 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 ornentum of the rabbit stained 

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

 plate of glass, rinsed with distilled water, treated for five minutes with 1 per 

 cent, nitrate of silver solution, again washed and exposed to the light in spirit. 

 \Vhen stained brown the spirit is replaced by oil of cloves. 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 focussed, 

 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 and an elastic 

 layer forming an inner coat, a middle coat of circularly disposed plain 

 muscular tissue, and a thin outer coat. The same differences also are 

 found between the arteries and veins, the walls of the veins being 

 tbinner and containing far less muscular tissue (fig. 116), and the 

 lining epithelium-cells, much elongated in both vessels, are far longer 



