Vin] THE CIRCULATORY SYSTEM 161 



Histology of the Heart. The wall of the heart is composed of 

 three layers, an outer adventitia, a middle muscular layer, and 

 an internal intima. 



The adventitia consists of a special fibrous connective tissue 

 resembling that of the fenestrate membrane of the compound 

 eye. It also encloses peculiar spindle-shaped cells and phagocytes. 

 It is separated from the muscular layer by a fine membrane. 



The muscular layer, which is the contractile portion of the wall, 

 is formed of complicated muscle-cells, partly fused into syncytia. 

 The cell-masses are so arranged that the heart is distinctly divided 

 into two lateral halves, by longitudinal dorsal and ventral zig-zig 

 lines, or sutures, and also by cross-sutures, in the form of half-rings 

 lying alternately to right and left of the longitudinal sutures. 

 The combined result is to divide the heart into hexagonal areas. 

 The muscle-fibres are striped, but there is no sarcolemma. 



The intima is a fine internal membrane lining the lumen of the 

 heart. It is almost certainly formed from the original sarcolemma 

 of the muscular layer. 



Innervation of the Heart. On either side of the heart there 

 runs a fine longitudinal nerve-cord, the alary nerve, derived 

 probably from the eighth ganglion of the ventral nerve-cord. 

 Each alary nerve gives off strong branches to the alary muscles. 

 Besides these, motor nerves derived from the nerve-supply to the 

 intersegmental muscles, and hence connected with each of the 

 ganglia of the ventral nerve-cord, enter the alary nerves at different 

 levels, and give off a rich network of nerves all over the wall of 

 the heart. 



Functions of the Heart. Besides the principal function of 

 pumping the blood forward in a closed stream, the heart has 

 been shewn to perform other duties. Thus, Zawarsin [203], by 

 injection of coloured substances into the blood, found that this 

 foreign matter was quickly removed by the phagocyte organs of 

 the hind-heart. Voinov [187] found that the entire wall of the 

 heart was capable of removing certain soluble impurities from 

 the blood, during its passage through the organ, and fixing it 

 temporarily in the adventitia, by means of the phagocytes. Thus 

 the heart is capable of performing a partial excretory function 

 (see p. 118). 



T. D.-F. 11 



