VASCULAR MUSCLE AND NERVES 



345 





B C 



no alteration in the diameters of the vessels, which may remain 

 for a considerable time in the state intermediate between excessive 

 dilatation and excessive constriction, which is commonly known as 

 tonus or tonic contraction of the vessels. 



Vascular rhythm, being much slower and less regular, does not 

 correspond with the functional rhythm of the heart, i.e. with the 

 alternation of cardiac systole and diastole. On the other hand, it 

 does correspond, and is in strict analogy, with the tonic rhythm 

 discovered by Fano in the walls of the tortoise auricle, and results 

 from the alternate contraction and expansion of the sarcoplasm of 

 the smooth spindle-shaped muscle cells of the tunica media, 

 particularly in the small arteries. 



The tone and rhythmical activity of the Veins is obscure, and 

 has been little studied; their 

 muscular cells are few in number, 

 and vary considerably in differ- 

 ent regions and tissues. The 

 veins of the bones and brain 

 have no contractile elements ; 

 the veins of the portal system 

 are highly muscular It is only 

 at the extreme ends of the venae 

 cavae and pulmonary veins, 

 where they open into the aur- 

 icles, that, in addition to a 

 perhaps slowly varying tonus, 

 we find any rapid and fairly 

 regular rhythmicalactivity,such 

 as gives rise to the systolic and 

 diastolic rhythm of the heart. 



The autochthonous activity of the vessels, whether tonic or 

 rhythmic, resembles that of the heart, in being automatic, i.e. 

 independent of external stimuli acting on the muscle cells. 

 This conclusion is reached, not merely by analogy, which in this 

 instance is of great value, but also by direct observation, from 

 which every unbiased observer must perceive that the oscillations 

 of tone in the vessels of the rabbit's ear are independent of 

 changes in external conditions. It is uncertain whether the 

 tonic and rhythmic activity of the vessels is, like that of the 

 heart, an inherent property of the muscle cells, as claimed by 

 the myogenists, or whether it is brought about by the peripheral 

 ganglia and nerve fibres which are particularly abundant in the 

 walls of the arteries, where they form a delicate plexus round 

 the smooth muscle cells. Experience shows that the neuro- 

 paralytic hyperaemia and rise of temperature consequent on the 

 division of all the nerves of the limb does not persist, but dis- 

 appears gradually, so that after some days the paralysed limb is 



FK;. 157. Tracings obtained with plethysmograph 

 of Fig. 1.06. (MOSSO.) The revolving drum is 

 moved on a short distance at each minute, so 

 that the writing point traces a horizontal line. 

 The variations in volume that occur during the 

 minute in which the drum is stationary are 

 indicated by vertical lines, which show the 

 extent of the oscillations in volume during that 

 period. In this case each cm. in height from 

 the ordinate corresponds to 2 c.c. of blood. At 

 P, P the subject replied to questions put by 

 the experimenter. At A he raised his left hand 

 to rub his nose, at B to rub his ear. At C he 

 heard the sound of an electric bell. 



