126 NORMAL HISTOLOGY. 



muscle-cells running in various directions. Outside 

 of this layer, and joining it to the muscular tissue 

 proper of the heart, is a layer of loose fibrillar con- 

 nective tissue, in which the blood, lymphatic ves- 

 sels, and nerves lie embedded. 



The valves of the heart consist of fibrillar connec- 

 tive tissue, arranged in membranes or fascicles, and 

 associated with elastic tissue, the latter being most 

 abundant at the free surfaces of the valves ; and 

 here it is present in the greatest quantity and 

 density on the surface which is most directly ex- 

 posed to the current of blood that is, on the 

 auricular surfaces of the tricuspid and mitral, and on 

 the ventricular surfaces of the aortic and pulmonary 

 valves. The elastic elements are also more abun- 

 dant in the valves of the left than of the right side 

 of the heart. The firmness and capacity for re- 

 sistance of the elastic tissue being borne in mind, 

 the significance of its distribution in the valves will 

 be readily perceived ; where they are the most ex- 

 posed to the impact and pressure of the blood, 

 there they are the most firm and dense. 



LYMPHATIC VESSELS. 



The larger lymphatic vessels have a structure 

 quite similar to that of the veins, and like the latter 

 they are supplied with valves. They approach the 

 arterial type, however, in that the muscular fibres 

 are quite abundant in proportion to the thickness 



