The heart of vertebrates is a chambered structure, the walls of which are composed of cardiac muscles or myocardium, held together by standards of connective tissue. The inner and outer surfaces of the heart are covered by layers of epithelial cells called endocardium and epicardium respectively.
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The entire heart is surrounded by a transparent covering called pericardium. The muscles of molluscan heart and of cephalopods and gastropods are of striated type.
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The ampuilary hearts are composed of spongy tissue of feebly striated endothelial cells.
The heart muscles of all animals exhibit some important physiological activities which are summarized as follows:
1. Excitability and contractility:
The heart muscles contract and expand rhythmically throughout life. These are effected by heat, chemicals, mechanical and chemical stimuli but while contracting there is no effect of any of these stimuli.
This non-responding period is called “absolute refractory period. When the contraction is over there is a brief period in which only a strong stimulus is effective, while the weaker ones have no effect at all.
2. Rhythmicity:
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These muscles keep on working rhythmically due to their-own property without the help of any external factor.
3. Conductivity:
The heart muscles are syncytial structure on account of which the stimulation received at any part of the wall of the heart spreads rapidly to all parts without stopping.
This conduction in mammalian heart is very swift and the bases for the swiftness are the bundles of His and Purkinje fibres.
4. All and none law:
The heart muscles do not contract when the strength of the stimuli is inadequate but contract maximally if the strength of stimuli is adequate. This property of heart muscles is known as all and none law.
5. Action of vagus on heart:
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The vagus nerve supplying the heart inhibits the heart movements and, thus, it balances the effect of sympathetic nerve which accelerates the heart beats.
