Volume 1

Green's Encyclopedia and dictionary of medicine and surgery / edited by J. W. Ballantyne.

Date:
1906-1909
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Green's Encyclopedia and dictionary of medicine and surgery / edited by J. W. Ballantyne. Source: Wellcome Collection.

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

    51/576 (page 33)
    AcanthoipeiyS.—A i^elvis with a bony tumour projectiug into it; pelvis spinosa. See TjABOur, Prolonged. Acanthosis Nigricans. &e Skin ; Nails, Afke('tions ok the. Acapnia!—The condition in wliicli there is a diminution in the amount of carbon dioxide in the blood ; true apncEa (Mosso). Acardia. — Congenital absence of the heart. See Teratology. Acarus Scabiei. See Scabies. Accelerator. — stimulator, e.r/. a nerve whose action increases the heart rate. See Physiology. Accentuation.—The state of being unnsiially distinct ; applied to the sounds of the heart. See Aorta, Aneurysm ; Heart ; Nephritis. Accessory.—Additional; accom])anying; contributing in a secondary degree. An accessory muscle, e.g. the accessorii orbicularis oris. See Nose, Accessory Sinuses ; Ovaries, Diseases OF ; and ToNsiLS, Diseases of. AccidentaB Haemorrhagre. — Hiemorrliage from the uterus during the last months of pregnancy, due to separation of a normally situated placenta. See Pregnancy, H.EMORIillAGE. Acclimatisation. See Climate. Accommodation. Introductohv ...... 33 Theories ....... 34 Nervous Mechanism . . . .35 Unequal Accommodation . . .35 Unequal Contraction of Ciliary Muscle 35 Amplitude ...... 36 Presbyopia 36 Accommodation and Convergence . . 37 Spasm ....... 37 Paralysis ...... 38 See also Brain, Physiology of; Glaucoma; Iris and Ciliary Bodies; Physiology; and Re- fraction. The refractive power of the dioptric media of the eye is not fixed; by an effort it can be increased, and when the effort is relaxed it returns to its previous state. This increase in the refractive power is termed accommodation. An individual with normal refraction is one who can focus parallel rays on his retina when the eye is at rest; he is thus able, without any effort, to see distinctly objects in the distance, since rays coming from the distance are practi- cally parallel. But in order to focus on his i-etina the diverging rays from objects near at liand, the refractive power of the eye must be VOL. I increased. When, therefore, he wishes to see near objects distinctly an effort of accommoda- tion has to be made. In order to obtain a clear idea of the opinions held on accommodation at the present time, it will be well to note first some of the chief anatomical relations of the parts concerned. The ciliary muscle lies in the outer part of the ciliary body, next to the sclerotic. Its fibres may be roughly divided into two parts—(a) an external set, whose course is longitudinal, attached in front to the fore part of the sclerotic between the canal of Schlemm and the anterior chamlier, and behind to the choroid opposite tlie ciliary processes and partly farther back towards the ora serrata; {h) an internal bundle, whose course is circular, running close to the insertion of the iris. In addition to these two sets, radial fibres exist which, starting from the sclerotic in company with the external longitudinal fibres, pass inwards to join the circular bundle. The crystalline lens, enclosed in its highly elastic capsule, consists of a central, more resist- ant part—the nuclevis—which does not alter its shape, and a jieripheral, semi-gelatinous zone —the cortex—which by pressure or traction can readily be made to change its form. The lens rests posteriorly in the hollow in the fore part of the vitreous bfidy, to which its capsule is closely adherent; it is also kept in position by means of the suspensory ligament. The suspensory ligament, consisting of a series of fine fibres radially disposed, is attached in front to the capsule of the lens, chiefly to the anterior surface near the margin, partly also to the margin and posterior surface. Behind it is adherent to the pars ciliaris retina; from the ora serrata to the most prominent parts of the ciliary processes. We are nOw in a position to consider the more important views ])ut forward to exjjlain the act of accommodation. Some of the older views need only be referred to briefly. At different times it has been thought that accommodation was due to— (1) Elongation of the GJohe by comliined com- pression and traction of the external and internal muscles of the eye respectively. Young proved that no elongation took place during accommo- dation by experiments on his own eye. (2) Alteration in curvature of the Cornea.— Young found that with his eye \inder water, tlius practically nullifying the refractive power of the cornea, his accommodation remained un- impaired. (3) Contraction of tlw Pupil.—Von Graefe's case of absence of iris due to traumatism, in wdiich there was full power of accommodation, shows this view to be incorrect. (4) Alteration in position of the Lens.—The lens moves forwards about '5 mm. during accom- modation ; but if it came forward so as to touch the cornea, the increase in the refractive power 3