164 THE ORGANS 



The larger fibres run in the connective tissue outside the adventitia. 

 From these are given off branches which enter the media, divide re- 

 peatedly, lose their medullary sheaths, and terminate mainly in the 

 media, although some fibres have been traced to their terminations in 

 the intima. 



TECHNIC 



(i) Capillaries, Arterioles, Small Arteries, and Veins. — Fix an entire brain, 

 or slices about an inch thick from its surface, in formalin-Miiller's fluid for twenty- 

 four hours (tcchnic 6, p. 7). Remove the pia mater, especially the thinner parts 

 which lie in the sulci between the convolutions, and harden in graded alcohols. 

 Select a thin piece, stain with haematoxylin (lightly) and eosin (strongly) (technic 

 I, p. 20), and mount in balsam or in eosin-glycerin. The veins, having thin 

 walls and being usually well filled with blood, appear distinct and red from the 

 eosin-stained red cells. The arteries, having thicker walls, in which are many 

 haemoglobin-stained nuclei, have a rather purple color. Between the larger 

 vessels can be seen a network of anastomosing capillaries with their thin walls 

 and bulging nuclei. Some are filled with blood cells; others are empty with 

 their collapsed walls in apposition. Note the appearance of an arteriole, first 

 focusing on its upper surface, then focusing down through the vessel. In this 

 way what is known as an "optical section" is obtained, the artery appearing 

 as if cut longitudinally. Trace the transition from arteriole to precapillary 

 artery and the breaking up of the latter into the capillary network. Similarly 

 follow the convergence of capillaries to form a small vein. 



(2) Instructive pictures of the relations of arteries, capillaries, and veins in 

 living tissues may be obtained by curarizing a frog, distending the bladder with 

 normal saline introduced through a small catheter or cannula, opening the abdo- 

 men and drawing out the bladder, which can then be arranged upon the stage 

 of the microscope. The passage of the blood from the arteries through the 

 capillary network and into the veins is beautifully demonstrated. ; 



(3) For studying the structure of the walls of a medium-sized artery and vein ■ 

 remove a portion of the radial artery, or other artery of similar size, and its accom- \ 

 panying vein, together with some of the surrounding tissues. Suspend the ves- ' 

 sels, with a small weight attached, in formalin-Miiller's fluid (technic 6, p. 7). [ 

 Sections should be cut transversely, stained with haematoxylin-eosin (technic 

 I, p. 20), or with haematoxylin-picro-acid fuchsin (technic 3, p. 21), and 

 mounted in balsam. The vessels of the adventitia — vasa vasorum — are con- 

 venient for studying the structure of arterioles and small veins. 



(4) Fix a piece of aorta in formalin-Miiller's fluid, care being taken not to 

 touch the delicate endothelial lining. Stain transverse sections with haema- 

 toxylin-eosin or with haematoxylin-picro-acid fuchsin and mount in balsam. 



(5) The outlines of the lining endothelial cells may be demonstrated as fol- 

 lows: Kill a small animal, cut the aorta, insert a glass cannula and, under low 

 pressure, thoroughly wash out the entire vascular system with distilled water. 

 Follow the water-by a one-per-cent. aqueous solution of silver nitrate. Remove 

 some of the smaller vessels, split longitudinally, mount in glycerin, and expose 



