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.

    55/576 (page 37)
    iiietropic, and therefore rather a stronger glass may be required, even though the amplitude of accommodation, being practically nil, can no longer change. It is clear that in hypermetropia, presbyopia will appear earlier, and in myopia later than in emmetropia ; indeed a myope of 4'5D does not need to wear glasses for close work. If, how- ever, the full correction of the H. or M. be worn, presbyopia is found to set in at the usual age. In astigmatism the correcting cylinder should be added, and in hypermetropia the correspond- ing power should be added to the presbyopic glass ; in myopia this should be subtracted. In ordering glasses for close work each case must be tried separately. Bonders' table of the amplitude of accommodation at different ages gives an indication, and a very accurate one, but individual variations exist. Thus hyper- metropes, with their well-exercised power of accommodation, often ])refer a much lower lens, and an individual older than his years prefers a stronger lens than one would expect. Accommodation and Convergence. — Witli each degree of convergence of the visual lines a certain effort of accommodation is associated ; but the connection is somewhat elastic, for without altering the angle of convergence the accommodation may to a certain extent be increased or diminished. This may be shown by looking at an object at about 2 m. distant, and holding first weak convex and then weak concave glasses before the ej-es, when the object can still be seen distinctly. The strongest con- vex and the strongest concave lens which can thus be used represent the measure of the amplitude of accommodation for this point. For each degree of convergence there is a rela- fit'e amplitwJe of acconiinoda.tion. The part of this amplitude of accommodation in use when looking at the object (without any glass), and represented by the convex lens, is called the negative part; while that represented by the concave lens is called the j^osttive part; it has not been brought into play, but is, as it were, held in reserve. It is essential for sustained accommodation at any angle of convergence that the positive part of the relative amplitude of accommodation be considerable. When the correcting glasses for an error of refraction are worn, it is obvious that the previous relation between convergence and accommodation is iipset; tliis probably accounts for nuich of the trouble felt in wearing correcting glasses for the first time. That the connection between con- vergence and accommodation is not rigidl}' fixed, is also shown by the fact that if weak prisms with their bases placed inwards or outwards be lield up before the eyes, when looking at an object, distinct vision can still be maintained. The prisms cause the visual lines to be more convergent or less convergent, accoirling as their bases are placed outwards or inwards, without altering the refraction. This shows that con- vergence of the visual lines may vary while tlie accommodation remains constant. Spasm of Accommodation.—In speaking of spasm of accommodation, three separate con- ditions must be distinguished—(a) tone of the ciliary muscle ; (b) hal>it spasm ; (c) true spasm. (a) Tone of the Cillari/ Mmrle.—In young people the instillation of atropin, liesides causing paralysis of accommodation, usually brings about a slight change in refraction; thus an emmetrope becomes slightly hypermetropic, hypermetropes and myojDes become slightly more hypermetropic and less myopic respec- tively. This is due to the ciliary muscle having lost a certain contraction which remained unre- laxed even with the eye at rest, and which is spoken of as tone of the ciliary muscle. In children this usually amounts to about TOD ; it diminishes with age, and is rarely found after thirty-five years, and, indeed, in some much younger people it may not be appreciable. {b) Habit Spasm. — In hypermetropes who have not worn correcting glasses, the habit of overcoming their hypermetropia by accommoda- tion is often so pronounced, that when the cor- recting glasses are first worn they cannot relax their accommodation. This condition may be termed h,abit s-jxmn. As a rule it is only com- pensatory, and is not in excess of the hyper- metropia ; and, moreover, in the dark room the spasm relaxes and a just estimate of the hyper- metropia may be made by retinoscopy or direct estimation ; but directly the individual makes the effort of looking at a distant object through the cori-ecting lenses the liabit spasm is set up. If the patient perseveres in wearing the glasses constantly, and especially if helped by instilla- tion of atropin and cessation of all close work for a time, the hal)it spasm soon subsides. In myopia, the constant habit of holding objects very close to the eyes sets up an exces- sive conti'action of the ciliary muscle; so that a stronger lens than that representing the myopia is selected. This tendency to over-correction must constantly be kept in mind when ordering glasses for myopes. The habit spasm here also is relaxed when the eyes are at rest in the dark room. When possiljle it is advisable to paralyse the accommodation with atropin or homotropin in all cases of hal;iit spasm before ordering glasses ; it ensures more accurate results, and the patients become more quickly accustomed to their glasses. (c) True Spasm.—No sharp line can be drawn between this condition and that termed habit spasm. Clinically, however, a distinct class of cases is met with, though rarely, in which the spasm of accommodation is excessive in amoiuit; so that emmetropes and those with low degrees of hypermetropia appear to be myopic, and