346 Journal of Agriculture. [8 JUNE, 1907. 



Death of muscle. — When an animal dies and the circulation stops, 

 the muscles continue to live for some time, as is shown by their contracting 

 on stimulation by electricity ; but as no oxygen is brought to them the 

 breakdown products, including lactic acid, accumulate and make the re- 

 action markedly acid. When these substances have been heaped up to 

 a certain degree, part of the protein of the muscle coagulates and this 

 coagulation produces a shortening of the muscle so that all the limbs of 

 the dead bodv become for a time quite rigid. This death stiffening is 

 termed rigor mortis. We may summarise the condition of a muscle in 

 rigor mortis by stating that it is more opaque than normal, is acid in re- 

 action, is in a contracted condition due to coagulation of certain proteins, 

 and is no longer responsive in anv way to stimulation — in other words is 

 dead. Then autolysis or self digestion sets in, the rigidity disappears, 

 and, in the case of an animal killed for food, the flesh becomes more 

 tender. Bacteria of course find autolysed muscle a suitable habitat and 

 will complete the breakdown, aided bv the larvge of various insects, if 

 the muscle (meat) be left exposed to the air. 



Heart Muscle. 



Heart muscle differs from skeletal chiefly in the fact that its con- 

 trations are rhythmic in character, are short and quick, and further, that 

 thev do not depend for their existence on the central nervous system. As 

 we shall see later, the nerve impulses that pass to the heart from the 

 central nervous system, only modify the rate or force of the contraction ; 

 if the nerves are cut, the heart continues to beat and, unlike skeletal 

 muscle, shows no sign of atrophy. Whether this power of beating 

 rhythmically is a property of heart muscle or is due to a nervous me- 

 chanism embedded in the heart substance, is still a debatable point. The 

 heart resembles skeletal muscle in using nitrogen-free carbon compounds 

 as sources of energy and in requiring a liberal supply of oxygen ; it also 

 contracts more forcibly the greater the resistance it encounters and it 

 passes into rigor mortis in much the same manner. It is unlike skeletal 

 muscle in being less dependent on the central nervous system, in not being 

 under the control of the will and in the much more restricted range in 

 the variation of its force. 



Involuntary or Smooth iNIuscle. 



Smooth muscle is peculiar in that it can remain almost indefinitely in 

 a tonic state of contraction without displaying fatigue. This state of 

 contraction is subject to rhythmic alterations. Both tonic contraction and 

 rhythmic change can exist if all connection with the central nervous 

 system is cut off; they are conditioned by an intrinsic nervous mechanism 

 or are actual properties of the muscle itself. Nerve impulses passing into 

 smooth muscle from the central nervous system can increase or diminish 

 the force of the contraction. These properties of smooth muscle make 

 it of service in the walls of arteries and veins and in hollow viscera like 

 the stomach, gut and uterus, &c., where sustained contraction is essen- 

 tial. It is also found in the iris, constricting or dilating the pupil ; 

 within the eyeball and concerned with the focussing of the lens ; and in 

 the skin producing ruffling of hair, fur or feathers. As one of its names 

 implies, it is not under the control of the will, but like the heart receives 

 nerve impulses via the autonomic system. 



