Recent work on the fundamental processes of regulation in biology (Ashby, 1956) has shown the importance of a certain quantitative relation called the law of requisite variety. After this relation had been found, we appreciated that it was related to a theorem in a world far removed from the biological – that of Shannon on the quantity of noise or error that could be removed through a correction-channel (Shannon and Weaver, 1949; theorem 10). In this paper I propose to show the relationship between the two theorems, and to indicate something of their implications for regulation, in the cybernetic sense, when the system to be regulated is extremely complex.

The anatomical evidence on the presence of efferent fibres in the optic nerve of vertebrates is scant and fragmentary. Herrick (1933) described in Amblystomaa tract of fibres which, arising in the posterior part of the nucleus preopticus, entered the optic nerve of the same and opposite sides. This tract had been described beforein fish by Holmgren (1920) and by Jansen (1929), both quoted by Herrick 1933. These authors had made their observations on the normal structures left after the enucleation of one eye. In 1948, however, Herrick says that he has been unable to confirm these observations in fish or his own in Amblystoma. Cajal (1889, 1933, 1952) and Polyak (1941) have described terminal arborizations around the amacrine cells in the retina of birds and mammals and have interpreted these fibres as terminals of efferent axons. Dogiel (1895) observed similar fibres in birds and was even able to follow them up to the optic nerve papilla. Physiological evidence for the presence of centrifugal fibres in the optic system has been provided by Arey (1916), who showed the influence of section of the optic nerve on the expansion and retraction of the pigmented cells of the retina in fish, and by Granit (1955 a, b), who demonstrated changes in the sensitivity of the retina after stimulation of the mesencephalic reticular system of the cat. Dodt(1956)hasidentified action potentials in centrifugal fibres in the retina after stimulating the optic nerve. More direct anatomical evidence has been provided by the finding of degenerationin the optic tract of birds by Wallenberg (1898), after lesions of the nucleus isthmoopticus. Armstrong (1951) found undegenerating fibres in the optic nerve of the snake, Natrix natrix, 133 days after section of the optic nerve. He interpreted these fibres as of non-retinal origin and probably of efferent character. Evidence of similar nature to Armstrong’s, gathered in the course of work on the functional and anatomical recovery of the optic nerve in amphibians after section, is here presented.