Note 203 — Chapter I · (continued 2)

See the interesting remarks of Sprengel (Hist. de la Médecine, vol. iv. pp. 271, 272), and Œuvres de Descartes, vol. iv. pp. 371 seq. What makes this the more observable is this: that the study of the crystalline lens was neglected long after the death of Descartes, and no attempt made for more than a hundred years to complete his views by ascertaining its intimate structure. Indeed, it is said (Thomson's Animal Chemistry, p. 512) that the crystalline lens and the two humours were first analyzed in 1802. Compare Simon's Animal Chemistry, vol. ii. pp. 419–421; Henle, Traité d'Anatomie, vol. i. p. 357; Lepelletier, Physiologie Médicale, vol. iii. p. 160; Mayo's Human Physiol., p. 279; Blainville, Physiol. comparée, vol. iii. pp. 325–328; none of whom refer to any analysis earlier than the nineteenth century. I notice this partly as a contribution to the history of our knowledge, and partly as proving how slow men have been in following Descartes, and in completing his views; for, as M. Blanville justly observes, the chemical laws of the lens must be understood, before we can exhaustively generalize the optical laws of its refraction; so that, in fact, the researches of Berzelius on the eye are complemental to those of Descartes. The theory of the limitation of the crystalline lens according to the descending scale of the animal kingdom, and the connexion between its development and a general increase of sensuous perception, seem to have been little studied; but Dr. Grant (Comparative Anatomy, p. 252) thinks that the lens exists in some of the rotifera; while in regard to its origin, I find a curious statement in Müller's Physiology, vol. i. p. 450, that after its removal in mammals, it has been reproduced by its matrix, the capsule. (If this can be relied on, it will tell against the suggestion of Schwann, who supposes, in his Microscopical Researches, 1847, pp. 87, 88, that its mode of life is vegetable, and that it is not ‘a secretion of its capsule’). As to its probable existence in the hydrozoa, see Rymer Jones's Animal Kingdom, 1855, p. 96, ‘regarded either as a crystalline lens, or an otolithe;’ and as to its embryonic development, see Burdach, Traité de Physiologie, vol. iii. pp. 435–438.