On a Non-striped Muscle of the Orbit. 321 
firmed the opinion of the structure already expressed. Tlie 
orbit of this animal differs from that of man in possessing 
much less perfect walls. As a consequence of this, the 
orbital membrane, or periosteum, is a structure of much im- 
portance, for it stretches across the floor of the orbit from 
its outer to its inner wall, extends backwards to the optic 
foramen, and completes the boundary of the cavity at the 
spot where the bony wall is w^anting. 
If the contents of the orbit be carefully removed, and the 
orbital membrane examined from above, it will be seen to 
be a well-defined structure, distinctly fibrous, and in many 
places having an almost tendinous-like aspect. Intimately 
connected with, and forming an essential part of it, is a thin 
layer of a pale reddish substance, which extends across the 
greater part of the floor of the orbit, passing backwards to 
the optic foramen and sphenoidal fissure. In close contact 
with this structure, especially at the posterior part of the 
orbit, is a well-marked vascular net-work, sufiiciently injected 
with blood to be distinctly visible. This vascular plexus 
constitutes a small rete mirabile, connected with the oph- 
thalmic artery. By removing a small portion of the reddish 
mass, teasing it out with needles under water, and examin- 
ing it with a quarter-inch objective, it may be seen to be 
composed for the most part of flat, pale, non-striped fibres, 
collected together in bundles, having a faintly granular 
aspect and exhibiting decided indications of nuclei in their 
interior. These bundles of flat fibres are mingled with 
ordinary fibrous tissue, both white and yellow, the latter be- 
coming more distinct after the addition of acetic acid. The 
pale, non-striped fibres have all the characters of the in- 
voluntary muscular fibre. Being desirous, however, of ascer- 
taining if these fibres could be resolved into their con- 
stituent fibre-cells, I adopted the plan which has been 
recommended by Beichert, and macerated a portion of the 
orbital membrane for forty-eight hours in dilute hydro- 
chloric acid. I then found that, by the aid of a very slight 
dissection, the fibres readily resolved themselves into the 
elongated fusiform cells of which they were composed. In 
no tissue which I have ever examined, consisting of the 
