REFLEX A CTION. 785 



or of few links only are characterised by " monotony "; the reactions which 

 employ long chains traversing many synapses are immensely variable. It is a 

 cardinal feature in the architecture of the nervous system that the longest 

 chains all include cerebral links. By rank in regard to nerve centres is under- 

 stood such position as relieves from local or merely segmental work and involves 

 general responsibility, i.e. for a series of segments or even for the whole body. 

 A touchstone for rank of centre in this nervous hierarchy is the degree to 

 which paths from separate loci and of differing sensual modality are confluent 

 thither. A low centre lies at the focus of afferent paths from a limited 

 locus and of one modality, e.g. from touch-spots in a small field of skin. 

 A high centre receives widely divergent multi-sensual paths, e.g. from organs 

 for touch, heat, cold, pain, and muscular sense for the hand as a whole. 



In addition to its power to modify the distribution of nerve impulses, both 

 in space and time, the nerve centre exhibits a valve-like function, allowing con- 

 duction to occur through it in one direction only. 1 ' This is called the "law of 

 forward direction " in the central nervous system. That the direction of 

 nerve impulses is not reversible along the neural chains, may be a function of 

 the " synapse." This seems a bridge open to traffic from nerve cell A to nerve 

 cell B, but barred to passage in the opposite sense. That this valve action of 

 the nerve centre is attributable to the synapse, is rendered likely by the 

 similar valve-like connection between motor nerve fibre and muscle cell and 

 between an efferent nerve and sense cell. How securely the circuits of the 

 nervous system are valved against regurgitation, is shown by the Bell-Magendie 

 law of the reactions of the spinal nerve roots. 



In reflex action at its simplest, it is difficult to demarcate between functions 

 of nervous and those of muscular or other contractile tissue. Muscular tissue, 

 like nervous, transmits a state of excitement induced in it at one point to 

 others, and even across extracellular boundaries. 



The reaction in which a local irritation of some point of the Stanniused heart 

 induces co-ordinate contraction of the whole myocardium, is akin to reflex 

 action. It is not yet clear whether the nervous or the contractile tissue is 

 really responsible for the reflexes of the medusa bell or of the manubrium of 

 Tiaropsis indicans,' 2 though cells resembling nervous have been demonstrated 

 in the Medusce (0. and R. Hertwig, 3 Schafer 4 ), as in the heart ventricle 

 (Heymans). 5 In the Ascidian, e.g. Ciona, 6 there is a characteristic " reflex " 

 nexus between oral and aboral vents. When one aperture is touched, both 

 shut. The musculature controlling each is of somewhat similar arrangement. 

 The main ganglion of Ciona lies between the apertures, and is the nervous tie 

 between the musculature of both. Yet, after ablation of the whole ganglion, 

 the reflex, although the threshold stimulus required is heightened, is still 

 elicitable. The nerve-path facilitates the reflex, but is not essential for it. 

 The "reflex" activity of the striped muscle of the intestine of the tench 7 

 seems altogether independent of the spinal cord. In the dog recent observa- 

 tions by Goltz and Ewald 8 show persistence of the " reflex " opening and 

 closure of the rectal and vesical sphincter, after complete removal of the 

 corresponding region of the spinal cord. 



The simplest reflex arc imaginable would be one in which a single 

 nerve conductor passed from some tissue recipient of environmental 

 stimuli to some other tissue responsive to the nervous excitement in 



1 W. James, "Boston Soc. of Nat. Hist., 1880," Scribners Mag., 1888; " Text -Book of 

 Psychology," London and New York, 1890, vol. ii. p. 580, etc. 



2 Romanes, Phil. Trans., London, 1877 ; "'Roy. Inst. Discourse," May 1877. 



3 "Das Nervensyst. u. die Sinnes organen d. Med.," 1878. 



4 Phil. Trans., London, 1878. 5 Arch, de pharmacod., Gand and Paris, 1896. 



6 J. Loeb, "Untersuch. u. physiol. Morphol. d. Thiere," Wurzburg, Bd. ii. 



7 Mahn, Arch. f. d. ges. Physiol., Bonn, 1892, Bd. Ixxii. S. 273. 



8 Ibid., 1896, Bd. Ixiii. 

 VOL. II. 50 



