578 APPENDIX. 



similar structure existed in them. In the orbit of the sheep I have 

 most satisfactorily observed appearances which have fully confirmed the 

 opinion of the structure already expressed. The orbit of this animal 

 difiers 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 importance, 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 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 network, suflSciently injected with blood to be 

 distinctly visible. This vascular plexus constitutes a small rete mirabile, 

 connected with the ophthalmic artery. By removing a small portion of 

 the reddish mass, teazing 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 indica- 

 tions of nuclei in their interior. These bundles of flat fibres are mingled 

 with ordinary fibrous tissue, both white and yellow, the latter becoming 

 more distinct after the addition of acetic acid. The pale, non-striped 

 fibres have all the characters of the involuntary muscular fibre. Being 

 desirous, however, of ascertaining if these fibres could be resolved into 

 their constituent fibre-cells, I adopted the plan which has been recom- 

 mended by Reichert, 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 resolve themselves 

 into the elongated fusiform cells of which they were composed. In no 

 tissue which I ever examined, consisting of the non-striped muscle, 

 have I succeeded in obtaining more beautiful and more perfect specimens 

 of the contractile fibre-cell than in this muscle of the orbital membrane. 

 The fusiform shape of the cells, their size, and the elongated rod-like 

 nucleus in the centre of each cell, gave to the texture a most character- 

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

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

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



