Bauhaus-Universität Weimar

Titel:
Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Person:
Helmholtz, Hermann von
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39649/181/
132, 133.] 
§12. Mechanism of Accommodation 
157 
tion in size of the image produced by this means would be counteracted by the 
forward displacement of the nodal point due to increase of e, as already ex¬ 
plained, undoubtedly, the reduction in size of the reflex in the posterior sur¬ 
face of the lens cannot be what it is seen to be unless there is some increase in 
the curvature of this surface, slight as it may be. 
When the values of q for the focal lengths of both of the schematic eyes of 
this section are computed, the results are 5.6051 for the case of far vision and 
5.3562 for the case of near vision, these magnitudes differing from each other 
by no more than about 1/21 of their average values, whereas the correspond¬ 
ing radii of curvature (6 and 5.5 mm) differ by 1/12. Here the change of the 
refracting medium partially conceals that of the radius of curvature and makes 
it appear to be smaller than it really is. The conclusion is that the posterior 
surface of the lens becomes more curved in accommodation for near vision. 
It is important for the mechanism of accommodation to know exactly 
the point of origin of the iris. In Fig. 3 Schlemm’s canal with its surrounding 
area was represented as it appears in very thin sections of the tunics of the 
eye. A is the cross section of the canal which probably forms an elongated 
cleft in the living eye accommodated for far vision; C is the cornea, S the 
sclerotica, D the conjunctiva, B the choroid, E a ciliary process and J the iris. 
The inner wall of the canal is made up of different tissues. The most posterior 
portion of this wall at a very evidently consists of the same type of closely 
interwoven white fibrous tissue as the sclerotica from which it arises. The 
anterior portion, on the contrary, consists of another type of tissue which is 
more opaque than fibrous tissue, and consists of fibres which are more sharply 
defined and are very resistant to the action of acetic acid and lime. Conse¬ 
quently, it may probably be considered elastic tissue. In front this portion of 
the wall is inserted between the membrane of Descemet and the cartilagi¬ 
nous substantia propria of the cornea. Behind it is partly attached to the 
posterior fibrous portion of the wall and partly unites with the fibre bundles of 
the tensor muscle of the choroid. The choroid is attached only to the posterior 
half of the inner wall of Schlemm’s canal at the point where the fibrous and 
elastic portions unite. However, from the anterior portion of the wall of the 
canal there originates also a loose network of fibres which exhibit the char¬ 
acteristics of elastic tissue and which are inserted on the periphery of the iris. 
The mass of fibres which belong to the tensor muscle and the iris seem to arise 
partly from the wall of the canal, though some seem to pass directly from the 
choroid to the iris. In the tissue of the ciliary processes a large number of 
sectioned blood vessels of wide lumen are visible and on their surface towards 
the vitreous humor is the layer of black pigment. 
In order to be convinced of the correctness of the structure of the iris as 
here stated, it is necessary, on the one hand, to examine thin sections of the 
dried tunics of the eye, taking into consideration the fact that the process of 
drying produces much distortion, and that the elastic fibres are very easily 
torn or broken away from the attachment of the iris when the latter is sepa- 
sated from the cornea. On the other hand, it is necessary to examine fresh 
specimens, in the preparation of which it is best to insert a bristle in Schlemm’s 
canal, at the same time carefully avoiding any traction on the iris or the 
choroid, for thereby the muscle mass which binds these parts together may be 
given any form at all. If the iris is lifted lightly and laid back on the ciliary 
processes, the fine elastic threads which pass over from it to the anterior edge 
of the canal become visible. If the bristle is then pulled forwards, the elas¬ 
ticity of the anterior portion of the canal wall will be evident. If, however, 
the iris and the choroid are turned forward and the bristle pulled backward, 
the posterior portion of the wall is seen to be inelastic. 
The author regards the method of attachment described above as very 
important with respect to the backward movement of the lateral portions of
        

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