CHAP, iv.] THE VASCULAR MECHANISM. 275 



Cardiac Muscular Tissue. The ventricle of the frog's heart is 

 composed of minute spindle-shaped fibres or fibre cells, each 

 containing a nucleus in its middle, and tapering to a point at each 

 end ; sometimes however the end is forked or even branched. 

 These fibres or fibre cells, in fact, resemble plain muscular fibres 

 save that they are somewhat larger and that their substance is 

 striated. The striation is due, like the striation of a striated 

 muscle fibre, to alternate dim and bright bands, but is rarely so 

 distinct as in a skeletal fibre ; it is very apt to be obscured by 

 the presence of dispersed distinct granules, which, in many cases 

 at all events, are of a fatty nature. Like the plain muscular 

 fibre, the cardiac muscular fibre has no distinct sarcolemma. 



A number of these fibres are joined by cement substance into 

 small bundles, and these bundles are, by help of connective tissue 

 which carries no blood vessels, woven into an intricate network or 

 sponge work, which forms the greater part of the wall of the 

 ventricle. Immediately under the pericardial coating, consisting 

 of a layer of epithelioid plates resting on a connective tissue 

 basis, the muscular tissue forms a thin continuous sheet, but 

 within this it spreads out into a sponge work, the meshes of which 

 present a labyrinth of passages continuous with the cavity of the 

 ventricle. The bars of this sponge work, varying in thickness 

 and, though apparently irregular, arranged on a definite system, 

 consist of bundles of muscular fibres united by connective tissue, 

 and are coated with the same endocardial membrane (flat epithelioid 

 plates resting on a connective tissue basis) that lines the cavity of 

 the ventricle and indeed the whole interior of the heart. The 

 cavity of the ventricle, in other words, opens out into a labyrinth 

 of passages reaching nearly to the surface of the ventricle. When 

 the ventricle is dilated or relaxed, blood flows freely into and 

 fills this labyrinth, bathing the bars of the sponge work, which, in 

 the absence of capillaries, depend on this blood for their nourish- 

 ment. When the ventricle contracts, the blood is driven out of 

 this labyrinth as well as out of the central cavity. Hence the 

 ventricle when dilated and full of blood is of a deep red colour, 

 when contracted and empty is extremely pale, having little more 

 than the colour of the muscular fibres themselves, which, like 

 striated fibres, possess in their own substance a certain amount of 

 haemoglobin or of myohsematin. 



The much thinner walls of the auricle consist of a much thinner 

 network of similar fibres united by a relatively larger quantity of 

 connective tissue into a thin sheet, with the pericardial mem- 

 brane on the outside and the endocardial membrane on the inside. 

 The fibres have in the auricle a much greater tendency to be 

 branched, and many, ceasing to be spindle-shaped, become almost 

 stellate. Among the obscurely striated, "but still striated fibres are 

 found ordinary plain muscular fibres which increase in relative 

 number along the roots of the veins, venae cavae and pulmonales, 



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