502 THE NERVOUS SYSTEM. 



tral artery, as it passes from the base of the ear toward its apex, divides 

 into branches supplying the capillary plexus ; and the vessels emerging 

 from this plexus unite in two principal veins, which return along the 

 edges of the organ toward its base. Both the central artery with its 

 branches, and the principal veins, are readily visible by transparency, 

 while the intervening tissue has a light rosy hue, from the blood in 

 the capillary circulation. The change in vascularity, first observed by 

 Schiff,* takes place in the following manner : The central artery dimin- 

 ishes in size, becoming narrower and fainter, until nearly invisible Its 

 branches disappear, the ear generally becomes more pallid, and the 

 veins, receiving from the capillaries a smaller quantity of blood, appear 

 less numerous and less distinct. The circulation in the organ is thus 

 reduced in quantity at least one-half. This condition lasts for eight or 

 nine seconds, after which the artery begins to enlarge. A thread-like 

 stream of blood enters it from below, increasing in thickness and capac- 

 ity, and extending rapidly upward into the arterial ramifications. The 

 tissues regain their rosy color, and the veins become prominent along 

 the edges of the organ. The artery is then in a state of diastole, sup- 

 plying the ear with a full quantity of blood. It remains in this condi- 

 tion for two or three seconds, when another contraction takes place, 

 and the circulation is again reduced. These alternations of constric- 

 tion and expansion recur usually about ten or twelve times per minute. 

 They are not strictly uniform either in extent or frequency, but they 

 take place with sufficient regularity to show that they are not accidental, 

 but depend on causes of internal origin. 



It is probable that other organs, if they could be examined by trans- 

 parency, would show a similar variation in vascularity. The small 

 saphenous artery in the rabbit has been seen by Loven, Kiegel, and 

 Yulpian to exhibit alternate movements of constriction and dilatation 

 once or twice per minute. In examining the circulation in the frog's 

 foot under the microscope, the small arteries sometimes show a local 

 constriction by which they are reduced in diameter for a certain time, 

 afterward enlarging to their former size ; and temporary changes of 

 vascularity in the glandular organs or the mucous membranes have 

 long been known to take place in connection with secretion or digestion. 



Contraction and Dilatation of Arteries under Nervous Influence. 

 When the sympathetic nerve is divided in the neck, one of the most 

 immediate and striking effects is a vascular congestion in the parts 

 above, on the corresponding side. This effect may be produced in 

 any warm-blooded animal, but is especially manifest in the ear of the 

 white rabbit, where the vascularity is easily examined by transparency, 

 and where the corresponding parts on the two sides can be directly 

 compared with each other. A few minutes after section of the nerve 

 all the vessels of the ear on the affected side become turgid with blood. 

 The artery enlarges, its branches become more visible, the tissues gen- 



* Comptes Kendus de 1'Academie des Sciences. Paris, 1854, tome xxxix., p. 508. 



