EQUALIZATION OF FLOW IN THE ARTERIES. 327 



destitute of those centres, but possessing a regularly-pulsating heart, 

 is another proof that the movements of this organ do not depend upon 

 a supply of nervous influence derived from them. Still they are 

 capable of being influenced by impressions transmitted through the 

 nerves. It has been ascertained by Valentin, that, after the heart has 

 ceased to beat, its contractions may be re-excited by stimulating the 

 roots of the Spinal Accessory nerve, or of the first four Cervical 

 nerves ; the influence of that stimulation being conveyed to the heart 

 by the Sympathetic system, the cardiac portion of which communicates 

 with these nerves. Irritation of the Par Vagum, also, has a tendency 

 to accelerate the heart's action, or to re-excite it when it has ceased ; 

 but the complete severance of both its trunks produces little disturb- 

 ance in the regularity of the movement. The action of the heart may 

 be also affected more directly through the Sympathetic system ; thus 

 it is excited by irritation of the cervical ganglia, especially the first ; 

 whilst continued pressure upon the cardiac nerve, by an enlarged 

 bronchial gland, has appeared to be the cause of its occasional suspen- 

 sion. It is without doubt through its nervous connexions, and probably 

 through the sympathetic system, that the heart receives the influence of 

 mental emotions. 



581. The movements of the heart may be suspended, or altogether 

 checked, by sudden and violent impressions on the nervous centres, 

 even though these do not occasion any perceptible breach of substance. 

 Thus in concussion of the brain, there is not merely insensibility, but 

 also a complete suspension of the circulation, occasioned by a failure of 

 the heart's power. This suspension may be permanent, so that ani- 

 mation cannot be restored ; or it may be temporary, as in ordinary 

 fainting. The well-known influence of blows upon the epigastrium, in 

 producing sudden death, is probably to be attributed to a similar cause, 

 namely, the shock thus communicated to the extensive plexus of gan- 

 glionic nerves, radiating from the semilunar ganglia, and proceeding to 

 the abdominal viscera. Violent impressions upon other nervous expan- 

 sions may produce a dangerous weakening of the heart's contractile 

 power ; this is the case, for example, with extensive burns, which may 

 produce faintness, and even death, especially in children, by the depres- 

 sion which they induce. Many other causes of sudden suspension of 

 the heart's action might be enumerated ; but they may be generally 

 traced to a strong impression upon the nervous system ; though of the 

 mode in which this operates we know nothing. 



4. Movement of the Blood in the Arteries. 



582. The Blood, thus propelled from the Heart into the Arteries by 

 a series of interrupted jets, would continue to flow in the same manner, 

 if it were not for the equalization of its movement, effected by the pro- 

 perties of the arterial walls. This influence is exerted by the middle or 

 fibrous coat, which consists in part of yellow elastic tissue ( 189), and 

 in part of non-striated muscular fibre ( 337). The proportion of these 

 two components varies in arteries of different calibre; the muscular 



