CHAP. XXVIII.] STRUCTURE OF THE VALVES OF THE HEART. 337 



that in other parts of the heart. This is due to an increased 

 development of the fibrous layer beneath the epithelium, espe- 

 cially of its yellow element. The precise meaning of the greater 

 thickness of endocardium in these parts of the heart is far from 

 obvious. 



Of the Structure of the Valves of the Heart. The valves of the 

 heart are formed by processes of fibrous tissue covered by epithe- 

 lium. Between the auricle and ventricle, as well as at the mouths 

 of the aorta and the pulmonary artery, there are remarkable deve- 

 lopments of fibrous tissue. Interposed between each auricle and 

 ventricle there is a fibrous zone or ring, to which the muscular 

 fibres of the auricle adhere on the one hand, and those of the 

 ventricle on the other. The fibrous tissue is prolonged inwards 

 towards the cavity of each ventricle, so as to form three curtains 

 on the right side (the tricuspid valve), and two on the left (the 

 mitral valve). These curtains are continuous with the chordae ten- 

 dineae, which appear to be inserted into them at their margins and 

 at various distances from them on the surface next to the ven- 

 tricle, but not at all on that which corresponds to the auricle. 

 They are covered on both 

 surfaces by epithelium, which 

 likewise extends over each of 

 the chordse teiidinese. The 

 fibrous tissue at the orifice of A portion of the aortic semilunar valve in the dog 

 the pulmonary [artery, as well tJS^*%* n ' b ' Nuclel of the e P ithelium 

 as at that of the aorta, con- 

 sists of fibrous cords arranged as three festoons continuous with 

 one another, the convexity of each being directed towards the 

 muscular tissue of the infundibulum of the ventricle, and its con- 

 cavity to the artery. On the right side the muscular fibres of the 

 infundibulum adhere to a p . igg 



small portion of the centre 

 of the convexity of each 

 of these festoons, and the 

 circular fibrous coat of the 

 artery seems closely at- 

 tached to the whole extent 

 of the concavity. On the 

 left side, the fibrous fes- Diafframof the semilunar valves of the aorta (after Mor . 



tnrm<s arp prvrmppfprl nnrtlv Rag)- a. Corpus [Arantii on the free border. 6. Attached 

 ,Q partly bor(Jer c . orifices of the coronary arteries. 



with the muscular fibres 



of the ventricle, and partly with the base of the inner curtain of 



