72 STRUCTURE OF THE VALVES. 



elastic fibres. These seem evidently meant to resist the distension which is 

 apt to occur when the heart contracts and great pressure is put upon the 

 endocardium. In all cases where high pressure is put upon walls composed of 

 soft parts, we always find muscular fibres present, and never elastic fibres alone. 

 No blood-vessels occur in the endocardium (Langer). 



The valves also belong to the endocardium both the semi-lunar valves 

 of the aorta and pulmonary artery, which prevent the blood from passing 

 back into the ventricles, and the tricuspid (right auriculo-ventricular) 

 and mitral (left auriculo-ventricular), which protect the auricles from 

 the same result. The lower vertebrata have valves in the orifices of 

 the venae cavse which prevent regurgitation into them; while in birds 

 and some mammals these valves exist in a rudimentary condition. 



The VALVES are fixed by means of their base to resistant fibrous 

 rings, consisting of elastic and fibrous tissue. They are formed of 

 two layers (1.) the fibrous, which is a direct continuation of the fibrous 

 rings, and (2.) a layer of elastic elements. The elastic layer of the 

 auriculo-ventricular valves is an immediate prolongation of the 

 endocardium of the auricles, and is directed towards the auricles. 

 The semi-lunar valves have a thin elastic layer directed towards the 

 arteries, which is thickest at their base. The connective-tissue layer 

 directed towards the ventricle is about half the thickness of the 

 valve itself. 



Muscular Fibres in the Valves. The auriculo-ventricular valves 

 also contain striped muscular fibres (Reid, Gussenbauer). Radiating 

 fibres proceed from the auricles and pass into the valves, which, when 

 the atria contract, retract the valves towards their base, and thus make 

 a larger opening for the passage of the blood into the ventricles; accord- 

 ing to Paladino, they raise the valves after they have been pressed 

 down by the blood-current. This observer also described some longi- 

 tudinal fibres which proceed from the ventricles to enter these valves. 

 There is also a concentric layer of fibres arranged near their point of 

 attachment, and directed more towards their ventricular surface. 

 These fibres seem to contract sphincter-like when the ventricle contracts, 

 and thus approximate the base of the valves, and so prevent too great 

 tension being put upon them. The larger chordse tendinise also 

 contain striped muscle (Oehl), while a delicate muscular network 

 exists in the valvula thebesii and valvula eustachii. 



Purkinje's Fibres. This name is applied to an anastomosing system of 

 grayish fibres which exist in the sub-endocardial tissue of the ventricles, especially 

 in the heart of the sheep and ox. The fibres are made up of polyhedral, clear cells, 

 containing some granular protoplasm, and usually two nuclei (Fig. 22). The 

 margins of the cells are striated. Transition forms are found between these cells 

 and the ordinary cardiac fibres ; in fact these cells become continuous with the 

 true fully developed cardiac fibres. They represent cells which have been arrested 



