94 NORMAL HISTOLOGY. 



VEINS. 



A vein also consists of three coats similarly named to those of 

 the artery. A vein differs from an artery in being thinner, and in 

 having a preponderance of connective over muscular and elastic tis- 

 sues. 



The tunica adventitia is composed of bundles of connective tissue, 

 elastic fibres, and involuntary muscle, which intercross, the general 

 arrangement being longitudinal. In the tunica media, the smooth 

 muscle fibres are circularly arranged and are associated with fibro- 

 elastic tissue. The tunica intima comprises the internal elastic 

 membrane, the subendothelial tissue, and a single layer of polyhe- 

 dral endothelial cells. 



The valves of the veins are produced by an infolding of the in- 

 tima. Their surfaces are lined with endothelial cells. 



CAPILLARIES. 



A capillary consists of a single layer of endothelial cells united 

 to each other by a cement substance and exhibits at intervals open 

 spaces, the stigmata. It is through the stigmata that the leucocytes 

 migrate on their mission to build up the worn-out tissues of the 

 body. Capillaries which supply nutrition to blood-vessels are called 

 vasa vasorum. The diameter of a capillary averages about 8 /*. 



Laboratory exercise No. 27. The heart. Fix and harden with alco- 

 hols, embed in paraffin, and stain with haematoxylin, method No. 6. 

 In examining your preparation, search first for the pericardium, and 

 demonstrate its endothelial lining- and fibrous connective tissue; then 

 examine the myocardium, observing the course of the fibres, their 

 branching and anastomosing, and the centrally located nuclei; finally, 

 examine the endocardium, composed of fibrous tissue, elastic fibres, 

 and endothelium. If possible, examine the structure of a valve. Search 

 for blood-vessels. Drawings. 



Laboratory exercise No. 28. Arteries and veins. Harden the great 

 aorta in alcohol, embed in paraffin, and stain with haematoxylin and 

 eosin, method No. 8. Examine with H. P. and make out the three coats 

 the tunica adventitia on the outside, tunica media in the middle, and 

 tunica intima on the inside. Name the structures which enter into 

 these coats. A study of veins may be made from sections of the um- 

 bilical cord, the lungs, or the tongue. Drawings. 



