APPENDIX. 579 



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 texture 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 Mam- 

 mals," describes it as distinctly fibrous, but possessing a considerable 

 quantity of a yellowish tissue, which he considers to be elastic, interpo- 

 lated with it. He regards the opinion, which had been previously 

 advanced by Gurlt, that the tissue was muscular, to be erroneous. 

 Stannius states that in those animals, in which the bony Avail of the 

 orbit is incomplete, the separation between the orbital cavity and the 

 temporal fossa is mostly effected by a fibrous membrane, containing also 

 abundant elastic tissue. He states that EtidoljM regarded these elastic 

 fibres to be muscular in bears, and that Meckel described a muscle in the 

 orbital membrane of Ornithorynchus. Chauveau speaks of the fibrous 

 membrane which completes the cavity of the orbit as entirely composed 

 of white inextensile fibres. Gurlt considers it to be a strong fibrous 

 membrane, with yellow elastic fibres interpolated. H. Midler, in a very 

 brief communication, states that he has found flat muscular fibres in 

 the inferior orbital fissure in man, and corresponding structures con- 

 nected to the membrana orbitalis of mammalia. 



It was supposed by those who held that the membrana orbitalis was a 

 highly elastic and not a muscular structure, that it was through its 

 elastic recoil that the eyeball was reprotruded in those animals which 

 retracted the ball through the contraction of a retractor muscle. H. 

 Midler, again, who speaks more positively than any who have preceded 

 him, not only of the existence of a muscle, but also of the kind of fibre 

 of which it is composed, considers that it antagonises those muscles which 

 retract the eyeball into the socket, and that thus the reprotrusion of 

 the globe is produced, not by a mere elastic recoil but by a muscular 

 contraction. 



If this hypothesis be correct, an arrangement exists in this locality, 

 which is certainly to be regarded as an unusual one — viz., an involun- 

 tary muscle acting as a direct antagonist to a voluntary muscle. 

 Whether the hypothesis be correct or not, I am disposed to consider that 

 the muscle has some special relation to the vascular arrangements in the 

 orbit. Its extension backwards to the foramina through which the 



