INFLUENCE OF THE QUALITY OF THE STIMULUS. 827 



removed, still shows reflexes in response to displacements of its limbs. 

 The spinal bird exhibits " struggling " movements in its dependent 

 limbs ; so similarly the spinal mammal. Though stimulation of 

 the skin is not thoroughly excluded in such observations, the exciting 

 stimulus is probably in chief part of the nature of muscular tensions and 

 positions. 1 When judged by effects on the rhythm of respiratory move- 

 ment (the transection being in the bulb), and on arterial blood pressure, the 

 faradisation of the central end of a purely muscular nerve in my experi- 

 ence, differs in its resultant reaction from that of a purely cutaneous, 

 chiefly in exciting a less intense reaction, i.e. quantitatively rather 

 than qualitatively. The mechanical stimulation of the muscles of the 

 rabbit is however said to invariably cause a slight fall in arterial 

 pressure, whereas the mechanical stimulation of the skin causes a rise. 2 

 Bradford 3 found that, as regards effect on blood pressure, the faradisation 

 of the central end of an intercostal nerve was far less efficient than 

 excitation of the parent trunk including the contribution from the 

 sympathetic, that is, including visceral afferent fibres. The visceral 

 afferent appears, therefore, to be peculiarly efficient for vasomotor 

 reflexes. 



When artificially applied " general " stimuli are employed, reflex 

 movement is usually i more easily evoked by excitation of the central 

 end of the afferent spinal nerve root than by excitation on the central 

 side of a severed peripheral nerve ; this difference is often very marked 

 indeed. But though it is easy to evoke spinal reflex movements 

 through the afferent roots and their filaments, it must be remembered 

 that the reflex so obtained is obtained under conditions unfitted to 

 exemplify reflex spinal movements of normal occurrence. The mammalian 

 spinal root is less a functional combination than a morphological one. To 

 excite the whole of a single afferent root is to produce a condition prob- 

 ably never under normal circumstances existent. The root reflexes — of 

 which a list is appended to this article — must, in order to be of use 

 for the understanding of the working of the cord, be considered in 

 relation with peripheral nerve reflexes, skin-spot reflexes, muscular 

 sense reflexes, etc. 



Among adequate stimuli, a class that are efficient for production of 

 reflexes but of nature as yet insufficiently determined by analysis, are 

 those which institute and determine many reflexes of posture. The re- 

 sumption of the right-side-up posture by the spinal eel, and by the headless 

 Amphioxus or Mantis or Grillotalpa, and even by the amputated ray of 

 Astcrias, are reactions to contact (stereotropic) or to gravitation 

 (geotropic) ; in the last-mentioned instance Loeb's 5 experiments indicate 

 stereotropism. Singer's reflex 6 spreading of the tail feathers of the 

 spinal bird is on a par with the above. In these and many other 



1 Reflex contraction of the gracilis can be regularly obtained by pinching the tibialis 

 anticus tendon (cat). Fairly quick passive extension of the hind-limb of the spinal dog 

 often causes activity (contraction) of the extensors of the same knee, sometimes of both 

 knees. This occurs when the skin has been desensitized. 



2 E. A. G. Kleen, Skandin. Arch. /. Physiol., Leipzig, 1890, Bd. i. S. 247. 



3 Joum. Physiol., Cambridge and London, 1889, vol. x. 



1 Sherrington, Phil. Trans., London, 1897, in the mammal, but in the bird Vulpian 

 has noted the converse (" Lecons sur le systeme nerveux," Paris, 18t>6. 



5 Arch. f. d. yes. Physiol., Bonn, 1891, Bd. xlix. ; Preyer, Mitth. a. d. zool. Stat, zu 

 Ncapcl, Bd. vii. S. 96. Also Loeb, " Eiuleitung in d. vergleich. Gehirnphysiol.," Leipzig, 

 1898. 



6 Vide supra, p. 814. 



