TFKNER ON THE ORBITAL AISTD KEEATOCEICOID MUSCLES. 107 



the orbit from its outer to its inner wall, extends backwards to the 

 optic foramen, and comj)letes 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 back- 

 wards 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 mira- 

 bile, connected with the ophthalmic artery. By removing a small por- 

 tion of the reddish mass, teazing it out with needles luider water, and 

 examining it with a quarter inch objective, it may be seen to be com- 

 posed 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 intei'ior. These bundles of flat 

 fibres are mingled Tvath ordinary fibrous tissue, both white and yellow, 

 the latter becoming more distinct after the addition of acetic acid. 

 The pale, non-striped fibres have aU the characters of the involuntary 

 muscular fibre. Being desirous however of ascertaining if these 

 fibres could be resolved into their constituent fibro-cells, I adopted 

 the plan which has been recommended by Belchert, 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 non-striped muscle, have I succeeded in 

 obtaining more beautiful and more perfect specimens of the contrac- 

 tile fibre-cell than in this muscle of the orbital membrane. The fusi- 

 form shape of the cells, their size, and the elongated rod-like nucleus 

 in the centre of each cell, gave to the texture a most characteristic 

 appearance. I may also mention, that when the orbital muscle in 

 the sheep Avas examined without the addition of any re-agent, besides 

 distilled water, a number of elongated rod-like nuclei were always 

 met with, lying free in the water surrounding the preparation, which 

 had evidently been loosened and detached during the dissection with 

 the needles. These nuclei corresponded in their characters to those 

 met with in the interior of the fibro-cells. The characters which I 

 have now enumerated render the muscular nature of the reddish tex- 

 ture connected with the orbital membrane sufficiently clear. 



On referring to the authorities who have written on the structure 

 of the orbital membrane I find that the following opinions have been 

 expressed concerning it. 



Bendz* in a paper " On the orbital membrane in the domestic 

 Mammals," describes it as distinctly fibrous, but possessing a consi- 



* Miiller's Archiv, 1841, p. 196. 



