216 



THE VASCULAR SYSTEM. 



The auriculo- ventricular opening (Fig-. 134 ^.) is a large 

 aperture guarded by two valves, a dorsal and a ventral : each valve 



AC. 



Fig. 136. 



PI 



LA. 



RA. 



TC. 



is formed by a redupli- 

 cation of the endothe- 

 lium, and contains a 

 small amount of con- 

 nective-tissue ; its free 

 margin is bound down 

 by a number (about 

 twelve) of fibrous cords, 

 the chordae tendineae, 

 which are attached by 

 their posterior extre- 

 mities to the trabe- 

 culae. 



d. The truncus ar- 

 teriosus (Figs. 132 TA., 

 134 ^.) arises at the 

 base of the ventricle, 

 on the right side of 

 its ventral surface, then 

 passes forwards and to the left, across the auricles, to reach their 

 anterior border near the median line, where it divides to form two 

 vessels. 



Internally it is incompletely divided into two compartments by 

 a spiral valve (Fig-. 134) ; the valve is attached to the dorsal surface 

 and is free ventrally : when the vessel is fully dilated (artificially), 

 the valve extends two-thirds of its diameter. The opening of the 

 vessel into the ventricle is g-uarded by two semilunar valves (Fig-, 

 134 ^.), the free margins of which are bound to the inner surface 

 of the tube by chordae tendineae. The distal extremity has a semi- 

 lunar valve in the left compartment (Fig. 134), which is attached 

 by a delicate band to the spiral valve (Ecker). 



C. The minute structure of the Heart. 



[The heart consists chiefly of muscl'>, but possesses also nerve- 

 fibres, nerve-cells, endothelium, and connective-tissue ; the truncus 

 arteriosus has in addition, blood-vessels. 



a. The muscular structure of the heart. 



(i) The arrangement of the muscle-fibres varies in the different 

 portions of the heart. That of the ventricle will be easily understood 



