64 ANATOMY OF THE RABBIT 



embryo, which are under the control of the sympathetic nervous 

 system, form the muscle coats of the dii^estive tube, and are 

 important not only for its repeated, peristaltic movements, but 

 also for its elasticity and expansive power. Smooth muscle is 

 also a constituent of many other visceral organs, especially glands, 

 in the active secretion of which it appears to play a mechanical 

 part. It is further distributed through the walls of the blood- 

 vessels, especially the arteries, where it forms the mechanical organ 

 of the vasomotor function. This consists in the control of the 

 diameter of the vessels by vasodilator and vasoconstrictor nerves 

 connected with the vagus nerve and the sympathetic nervous 

 system. The constrictive action is stimulated by secretion of the 

 suprarenal glands. Such regulation of the vessels is important, 

 first, in maintaining tone of the vessel-walls and therefore blood- 

 pressure, and, second, in controlling loss of heat from the surface of 

 the body. Action of the vasomotor nerves may be demonstrated 

 physiologically in a variety of ways. Transection of the cervical 

 svm pathetic nerve of one side in the living rabbit is followed by 

 vasodilatation of the ear, the congestion of which can be seen, and 

 the heat loss is demonstrable by feeling with the hand. Stimulation 

 of the cut end which is attached to the head is followed by vaso- 

 constriction. 



Muscle of the Heart 



Cardiac muscle, most nearly allied in action to smooth muscle, 

 is the mechanical organ of the rhythmical contraction or beat of 

 the heart. The contraction takes place according to the succession 

 of the chambers or the course of the blood, and the rate and strength 

 of the beat are regulated by a minute mass of highly specialized 

 tissue, the sinu-atrial node, imbedded in the wall near the entrance 

 of the right superior caval vein. A second, similar mass, the atrio- 

 \entricular node, receives the impulse from the first and transmits 

 it through a band of conducting tissue to the muscle of the ventricles. 

 The excised heart in the case of lower vertebrates continues to 

 beat for some time automatically or under stimulation. This 

 behaviour has been interpreted as purely automatic action of the 

 heart muscle, but may depend upon intracardiac nerve connections. 

 The rate and strength of the beat in the intact animal may be 

 modified through the vagus and the sympathetic nerves, the 



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