FUNCTIONS OF THE SPINAL CORD. 635 



ena. From experiments on frogs and other animals Pfluger has laid down 

 the following " laws " of reflex action : 



1. The Law of Unilateral Reflection. In accordance with this law it follows 

 that when in consequence of irritation of one or more sensory nerves move- 

 ment takes place on one side, it is always on the same side as that to which 

 the irritation was applied. 



2. The Laic of Symmetry. It may further be demonstrated that when the 

 impression is sufficiently strong to be irradiated to the muscles of the oppo- 

 site side, these are always congenerous with those excited to action on the 

 irritated side. 



3. The Law of Intensity. The movements on the irritated side are some- 

 times equalled in intensity by those of the opposite side, but if they are not 

 equal they are always of inferior vigor. 



4. Law of Reflex Irradiation. Lastly, in the Spinal Cord, the motor nerve 

 excited is always on the same plane as the sensory nerve that has been irri- 

 tated, but if the sensory irritation be so powerful as to be propagated beyond 

 this point, it always affects the nerves above and never those below. In the 

 case of the reflex movements due to the encephalon, however, the irradia- 

 tion takes place from above downwards towards the Medulla Oblongata, 

 which possesses the power of generalizing all reflex movements; whenever 

 the irradiated excitation in its upward course reaches the Medulla Oblon- 

 gata, all the nerves arising from the encephalon are affected. 



It is noticeable also that the reflex activity is greatest in young animals. 

 It varies in the different classes of animals, appearing to be most strongly 

 developed in Birds, then in Mammals, and least in Fishes, It is exalted by 

 opium, alcohol, and nicotin, and especially by strychnia. It is diminished 

 by acetate of zinc, chloral hydrate, hydrocyanic acid, belladonna, digitalis, 1 

 and bromide of potassium. 2 Like other parts of the nervous system the 

 cells ministering to reflex action can be exhausted by effort and require rest 

 to recover their properties. The experiments of Iviissmaul and Tenner 3 

 show that interruption of the blood-supply, contrary to what might be ex- 

 pected, has a tendency to exalt the reflex excitability of the spinal cord, 

 though to a much less extent than the nervous centres in the Brain and 

 Medulla Oblongata. Complete suppression of the blood-supply to the Spiual 

 Cord rapidly causes complete paralysis. [Setschenow has discovered that 

 nerve-centres which inhibit reflex action are seated mainly in the thalami 

 optici and corpora quadrigemiua. 4 ] Reflex acts are greatly enfeebled, or 

 are altogether absent, when the blood is overcharged with oxygen, so that 

 the reflex contractions consequent on the administration of strychnia, brucia, 

 thebaia, and caffein are suppressed in animals to which they have been given 

 by vigorous artificial respiration. 5 The time occupied in the performance of 

 a reflex act is dependent upon the strength of the stimulus, being very rapid 

 when the stimulus is powerful. 6 Exner, experimenting on the reflex act of 

 winking (the movement of the lids being excited by the application of a 

 spark to the cornea), found that with a weak spark the time in man was 



1 Weil, Central blatt, 1871, p. 839. 2 Meihuisen, Centralblatt, 1873, p. 88. 



3 On the Nature and Origin of Epileptiform Convulsions, New Syd. Soc. Trans- 

 lation, 1859, p. 54. 



4 [Physiologische Studien uber die Hpmmungs-mechanismen fiir die Rt'fiexthiitig- 

 hoit des Riickemnarks im Gehirne des Frosches, von Dr. J. Seti-chenow. Neue "Wr- 

 suche am Him und Kiickenmark des Frosches, von Dr. J. Setschenow und B. Pas- 

 chutin.] 



5 See Leube and Uspensky, Centralblatt, 1868, pp. 41 and 077; Uspensky, Keichert 

 and Du Bois-Reymorid's Archiv, 18G8, p. 401. 



6 Rosenthal, Studien uber Reflexe, Monatsbericht d. Berlin. Akad., 1873, p. 104 ; 

 and Exner, Pfliiger's Archiv, 1874, p. 526. 



