In the case of simple antagonistic muscles, and in the instances of simple spinal reflexes, the shifts of conductive pattern due to interaction at the mouths of common paths are of but small ex tent. The co-ordination covers, for instance, one limb or a pair of limbs. But the same principle extended to the reaction of the great arcs arising in the projicient receptor organs of the head, e.g., the eye, which deal with wide tracts of musculature as a whole, operates with more multiplex shift and wider ambit. Releasing forces acting on the brain from moment to moment shut out from activity whole regions of the nervous system, as they conversely call vast other regions into play. The resultant singleness of ac tion from moment to moment is a keystone in the construction of the individual whose unity it is the specific office of the nervous system to perfect. The interference of unlike reflexes and the alliance of like reflexes in their action upon their common paths seem to lie at the very root of the great psychical process of "attention." The spinal cord is not only the seat of reflexes whose "centres" lie wholly within the cord itself ; it supplies also conducting paths for nervous reactions initiated by impulses derived from afferent spinal nerve, but involving mechanisms situate altogether head ward of the cord in the brain. Many of these reactions affect con sciousness, occasioning sensations of various kinds.
Besides the paths followed by headward-running impulses the spinal cord contains paths for impulses passing along it back wards from the brain. These paths lie almost entirely in the ven trolateral columns of the cord. The fibres of which they are com posed cross but little in the cord. Their sources are various, some come from the hind brain and some from the mid brain, and in the higher mammalia, especially in man and in the anthropoid apes, a large tract of fibres in the lateral column (the crossed pyramidal tract) comes from the cortex of the neopallium of the fore brain. This last tract is the main medium by which impulses initiated by electrical stimulation of the motor cortex reach the moto-neurones of the cord and through them influence the activity of the skele tal muscles. Of the function of the other tracts descending from
the brain into the cord little is known except that mediately or immediately they excite or inhibit the spinal motoneurones by various levels. How they harmonize one with another in their action or what their purpose in normal life may be is at present little more than conjecture. Such terms, therefore, as "paths for volition," etc., are at present too schematic in their basis to war rant their discussion here. (C. S. S.) BIBLIOGRAPHY.—M. Philippson, Tray. d. Lab. d. Physiol. Institut Solvay (Bruxelles, 1905) ; H. Head and Theodore Thompson, Brain, vol. xxix., p. 537 (1906) ; C. S. Sherrington, Integrative Action of the Nervous System (London and New York, 1906) ; Graham T. Brown, "Studies," Quart. _loam Exp. Physiol., vols. iv.–vii. (1911-13) ; A. Forbes (with others) Amer. Journ. of Physiol., vol. xxxi.–vol. lxxii. (1912-27) ; J. S. Beritoff, Quart. .101.072. of Exp. Physiol., ix., p. 199 (1915) ; G. Riddoch, Brain, xl., p. 264 (1918) ; L. H. Weed, Am. Journ. of Physiol., xliii., p. 131 (1917) ; Cornelius Winkler, Opera Omnia tome vi. (Haarlem, 1918) ; F. M. R. Walshe, Brain, lxii., p. (1919; and L'Encephale, xx., p. 73 (1925) ; G. Riddoch and F. Buzzard, Brain lxiv., p. 397 (1921) ; E. D. Adrian and J. M. D. Olmsted, Journ. of Physiol., p. 426 (1922) ; C. Bazett and W. G. Penfield, Brain, xlv., p. 185 (1922) ; A. Forbes, Physiol. Review, ii., pp. (1922) ; F. Bremer, Arch. Intern. de Physiol., xxi., p. 3o8 (1923) ; E. G. T. Liddell and C. S. Sherrington, Proc. Roy. Soc. B., vol. xcv.– xcvii. (1923-25) ; G. Rossi, Arch. di Fisiol., xxi., p. 275 (1923) ; F. Tilney and H. F. Riley, Form and Function of the Nervous System (New York, 1923) ; F. Bremer, et P. Rylant, C.R. Soc. Biol., vol. xci.–xcii. (Bruxelles, 1924-25) ; S. Cobb, Physiol. Rev. 5, p. 518 (1924) ; R. Magnus, Proc Roy. Soc., 98B., p. 339-353 (1925) ; L. Ballif, J. F. Fulton and E. G. T. Liddell, Proc. Roy. Soc., 98B., p. 589 (1925) J. F. Fulton, Muscular Contraction of the Reflex Control of Move ment (Baltimore and London, 1926).