392 A MANUAL OF VETERINARY PHYSIOLOGY 



by the spread of the stimulus from one fibre to another, but as 

 the result of all the fibres being stimulated simultaneously. 

 There are nerves and ganglion-cells in abundance in pale muscle ; 

 the nerves, which are chiefly non-medullated, form a fine plexus, 

 with the ganglion-cells placed at the junctions of the plexus. It 

 is probably due to these cells that involuntary muscle continues 

 to contract when all connections with the centre are destroyed, 

 though some physiologists see reason for thinking that the con- 

 traction of pale muscle may be carried out just like that of heart 

 muscle — viz., as a self-acting mechanism, independent of any 

 nervous connections. 



The nerve supply of involuntary muscle is peculiar, and 

 presents a great contrast to that of red muscle ; whereas the 

 latter only receives one variety of motor supply, pale muscle 

 receives two — viz., one set of fibres which stimulates contrac- 

 tion, and another which inhibits it. Both sources are derived 

 from the sympathetic system, which again is in great contrast 

 to the arrangement of the nerve supply to red muscle. 



Heart Muscle, as we have seen (p. 31), is in structure both red 

 and striated, nevertheless it is involuntary ; the fibres are char- 

 acterised by being formed of branched, nucleated, quadrilateral 

 cells, while the sarcolemma is absent. The contraction of the 

 heart is primarily dependent on the properties of its muscle 

 substance, though the automatism is carefully directed by 

 nervous mechanisms (p. 48) . 



Muscular Contraction. — This apparently simple act is ex- 

 tremely complex, and will require to be dealt with in some little 

 detail. 



Muscles are tissues possessed of irritability and contractility — 

 viz., they possess the power of responding by a movement to the 

 application of a stimulus. The normal stimulus is effected 

 through the motor nerves under the control of the brain or spinal 

 cord,but of the nature of this stimulus we are ignorant. A coarse 

 reproduction of it can be effected by pinching, pricking, chemical, 

 thermal, or electrical stimuli, applied to either the nerve or the 

 muscle itself, and to all these the three varieties of muscle are 

 responsive. When a muscle contracts, in addition to becoming 

 shorter and thicker, it undergoes changes in its extensibility, 

 elasticity, and temperature ; there are also alterations in its 

 electrical condition and chemical composition. These can all be 

 studied by employing a muscle of the frog, which retains its 

 irritability for a long time after removal from the body. Such a 

 muscle suitably prepared is known as a muscle-nerve preparation, 

 and with certain modifications what is found to occur in this as 

 the result of contraction, occurs also essentially in the living 

 mammalian muscle. 



