Manual of physiology for students and practitioners / by H. Willoughby Lyle.

  • Lyle, H. Willoughby.
Date:
1911
Catalogue details

Licence: In copyright

Credit: Manual of physiology for students and practitioners / by H. Willoughby Lyle. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    670/776 (page 648)
    is the direct light reflex, and the course of the impulse is as follows : The rays of light traverse the eyeball, and are brought to a focus on the retina; here they travel through the layers of the retina from within outwards, until they fall on the receptive part of the retina which is the specialised sensory nerve-epithelium; this includes the rods and the rod fibres, the cones and the cone fibres. The function of these is to convert the light stimulus into an afferent nerve impulse, which travels back through the retina from without inwards, passing through the first neurone in the internal nuclear layer of the retina, thence to the second neurone or the ganghon cell, and from the ganghon cells through the nerve fibres back along the optic nerve to the anterior corpus quadrigeminum. In the anterior corpus quadrigeminum the third neurone starts, from which the nerve impulse passes to that part of the third nerve nucleus which innervates the sphincter pupills muscle. The impulse passes from these cells along the third nerve, through its lower division to the ciliary ganghon, through this cell station and by way of the short ciliary nerves to the sphincter pupillae; hence the contraction of the pupil takes place. 2. If rays of light traverse the media of one eye, not only the pupil of that eye contracts, but also the pupil of the opposite eye. This is the consensual light reflex; it is brought about by the cells in the part of the third nerve nucleus which form the centre for the sphincter pupillce, and which constitute a single nucleus. In all probability this single collection of cells innervates both sphincters pupillse. 3. When the eyes are focussed at a near object; this focussing is the accommodative synkinesis. 4. When the eyeballs converge. 5. During sleep (in this state, as a rule, most sphincters remain contracted). 6. On direct stimulation of the optic nerve, the anterior corpus quadrigeminum, the anterior part of the third nerve nucleus, or the third nerve itself. 7. After section or paralysis of the cervical sympathetic nerve. 8. Under the influence of drugs, some of which act upon the centre for the contraction of the pupil, and others act peri- pherally,—e.g. opium and morphia produce bilateral myosis ; this is due to central action. Pilocarpine, eserine, and muscarin