On a Non-striped Muscle of t lie Orbit. 321 



firmed the opinion of the structure already expressed. The 

 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 wanting. 



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, sufficiently 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 ReicJiert, 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 



