CIRCULATION OF THE BLOOD. 



553 



detected. Anatomical data, therefore, support the view — which would 

 otherwise be rendered probable, as, for example, in the production of 

 blushing from emotions — that the calibre of the minute arterioles is under 

 the control of the central nervous system. Numerous experiments still 

 further demonstrate the truth of this statement. When the web of a 

 frog's foot is examined under the microscope it will be found that the 

 smaller arterioles are constantly varying in calibre, sometimes beino- so 



contracted — evidently due to the contraction of their muscular fibres 



as to almost shut off blood from the part supplied by the contracted 



Fig. 234.— Cardiac Plexus and Stellate Ganglion of the Cat. {Landois. ) 



R, right; J.. I--i't XI 1 ^- 1, vagus: V, cervical sympathetic, and in the annulus of vieusaens : 2, 

 communicating branches from the middle cervical ganglion and the stellate ganglion ; 2", thoracic sym- 

 pathetic ; 3, recurrent laryngeal; 4. depressor nerve; 5, middle cervical ganglion; .V, communication 

 between 5 and the vagus ; 6, stellate (first thoracic) ganglion : 7, communicating branches with the vagus ; 

 8, accelerator nerve ; 8, 8', 8", roots of the accelerator nerve ; 9, branch of the stellate ganglion. 



vessel, at other times so dilated as to cause the tissues supplied by the 

 vessel to become gorged with arterial blood. If the web of the frog's 

 foot be examined and an arteriole picked out which appears to be midway 

 between the states of extreme relaxation and extreme dilatation, and a 

 weak induction current be then applied to the sciatic nerve, the arterioles 

 will all, as a rule, be found to become immediately contracted. If the 

 effects of stimulation be allowed to pass off, and then, instead of stimu- 

 lating the sciatic, this nerve be divided, directly opposite results will 



