356 PHYSIOLOGY CHAP. 



lateral pressure of the aorta after ligaturing the crural vein. 

 Directly after the injection he observed either an instantaneous 

 fall of blood pressure, or a fall preceded by a rise. Heger referred 

 this reflex phenomenon to the action of the capillary walls, since 

 neither pressor nor depressor effects appeared, when the injected 

 substance was localised in the artery or vein. The reflex also 

 occurs when the hind-limb is only connected by the sciatic to the 

 rest of the body. 



Spallitta and Consiglio, in Marcacci's laboratory, extended and 

 completed the observations of Heger. According to the Italian 

 workers, excitation of the sensory nerves to the internal surface 

 of the blood-vessels, by chemical agents, constantly produces not 

 merely rise of blood pressure, but also slowing of heart-beat with 

 increased amplitude of pulse. Since excitation of the sensory 

 nerves of the heart, or, according to the latest researches, of the 

 aorta, produces a depressor effect, antagonistic, i.e. to that of the 

 afferent nerves to the peripheral vessels, it seems logical to conclude 

 that under normal conditions also the two kinds of sensory nerves 

 exert an opposite action on the circulation. The sensory nerves of 

 the heart or aorta prevent undue filling or overloading of the 

 heart ; the sensory nerves of the peripheral vessels obviate undue 

 filling and distension of the vessels. 



Since it is proved that vascular reflexes may be excited by 

 afferent fibres in the walls of the vessels themselves, it is probable 

 that they play a part in the physiological regulation of blood 

 pressure. On the other hand, the tone of the vessels can be 

 reflexly influenced by any centripetal nerve. Localised vascular 

 reflexes usually cause the vessels to widen, but at other times 

 they are first narrowed, and subsequently, as an after-effect, 

 become wider. This means that the stimulation of the afferent 

 nerves is transmitted either by the vaso-dilators, or by the vaso- 

 constrictors, or lastly, by both kinds of nerves. The inconstant or 

 totally negative results of a number of experiments find their 

 probable explanation in the partial or total inhibition of the effect. 

 Only a few examples can here be cited. 



If the nerves of taste are excited by sapid substances, there is 

 not only a reflex secretion of saliva, but also a marked dilatation 

 of the vessels innervated by the chorda tympani (Claude Bernard). 

 Central excitation of the posterior auricular nerve usually produces 

 dilatation of the vessels of the rabbit's ear ; often, however, this is 

 preceded or followed by vaso-constriction (Snellen and Loven). 

 On stimulating the nerve in the dorsurn of the dog's paw, dilata- 

 tion of the saphenous artery is obtained ; on exciting the infra- 

 orbital, or mental (labial) nerve, there is dilatation of the external 

 maxillary artery (Loven). Central excitation of the sciatic, 

 the vagus, or the posterior spinal roots, usually produces 

 reflex constriction of the kidney vessels ; on the other hand, 



