Professor Donders, of Utrecht,[14] well known as one of the highest
authorities in Europe on vision and on the structure of the eye,
has most kindly undertaken for me this investigation with
the aid of the many ingenious mechanisms of modern science,
and has published the results.[15] He shows that during
violent expiration the external, the intra-ocular, and the
retro-ocular vessels of the eye are all affected in two ways,
namely by the increased pressure of the blood in the arteries,
and by the return of the blood in the veins being impeded.
It is, therefore, certain that both the arteries and the veins
of the eye are more or less distended during violent expiration.
The evidence in detail may be found in Professor Donders'
valuable memoir. We see the effects on the veins of the head,
in their prominence, and in the purple colour of the face
of a man who coughs violently from being half choked.
I may mention, on the same authority, that the whole eye
certainly advances a little during each violent expiration.
This is due to the dilatation of the retro-ocular vessels,
and might have been expected from the intimate connection of
the eye and brain; the brain being known to rise and fall with
each respiration, when a portion of the skull has been removed;
and as may be seen along the unclosed sutures of infants' heads.
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