I believe it is universally assumed by English physiologists that the zero of accommodation is naturally associated with parallel visual axes as in the “Primary Position” of Helmholtz, Listing, &c.

It is self-evident that when even in actual life a body is viewed at infinite distance the visual axes must be parallel. It is also well known that the nervous connection between convergence and accommodation is a most delicate and susceptible one, and is none the less so naturally because it is capable of being overcome for a time by various conditions. It is therefore quite reasonable to suppose that when the ciliary muscle is at rest the converging mechanism should be so likewise; and to expect that the invariable association of the visual actions of a life-time should be impressed, if not at birth, as Porterfield suggested, “by an original, connate, and immutable law,” at least by “dint of habit,” upon the very constitution of the governing ganglia.