140 CAUSE OF THE MOTIONS OF THE HEART. 



Cause of the kl oc) d> or an 7 re fl ex action arising from the cerebro-spinal 

 motions of the system, but must be attributed to the organ itself, is proved 

 by their continuance after its excision from the body, or even 

 after it has been cut in pieces. Some have supposed that the minute 

 sympathetic ganglia with which it is furnished are the source of the mo- 

 tive power ; others are disposed to impute it to a self-contractile power 

 of its muscular fibres, irrespective of any nervous agency. Of course, it 

 is admitted by all that the brain and spinal cord can influence these 

 movements, but such effects are superadded and not uniform. 



Of these opinions, we shall find many reasons for preferring the first 

 when we come to the description of the nervous mechanism. It will be 

 then seen that one of the prominent functions of nervous ganglia of a cer- 

 tain order, and particularly the ganglia of the sympathetic, is the storing 

 up of impressions they have received, and thus becoming reservoirs or 

 magazines of force. The power thus engendered or contained in them 

 is by no means always delivered out in totality at once, but it may be 

 in small portions, at intervals, for a long time ; and doubtless in this 

 way the minute sympathetic ganglia of the substance of the heart retain 

 a power of keeping up the motions of that organ for a certain period of 

 time, even though great lesions or morbid changes may have supervened. 

 Such a mechanism recalls the manner in which chronometers are kept 

 going during the short time that the action of the main-spring is taken 

 off when the watch is wound up. 



2d. The arteries are tubes consisting of different tunics or layers va- 

 Description of rioTisly numbered by anatomists, but which may be suffi- 

 the arteries. ciently described as, 1st. The exterior tunic, containing fibres 

 generally running lengthwise, connective and elastic tissue : it is of about 

 the same thickness as the tunic below ; 2d. The middle tunic, character- 

 ized by being composed of non-striated muscular fibres circularly ar- 

 ranged ; 3d. The interior tunic, which is thin, and consists of a cellular 

 or epithelial layer, smooth and polished, to permit of the ready passage 

 of the blood. 



The elasticity of the arteries enables them to sustain the sudden action 

 of the heart by distending to a certain degree as the blood is driven into 

 them, and by their gradual collapse when the ventricles cease their pres- 

 sure, the jetting or intermitting flow is converted eventually into a con- 

 tinuous stream. The mechanical influence of the heart is thus decom- 

 posed into two portions : one, which is of momentary duration, or, at all 

 events, lasting only so long as the ventricle contracts ; and a second, 

 which is occupied in distending the elastic arterial tube; but this por- 

 tion is not lost to the circulation, since the tube, as it contracts, yields 

 it back again to the blood. The momentary impulse of the heart is thus 

 spread over a considerable duration without loss. 



