1894.] TELBOSTEAlf MORPHOLOGY. 427 



are the solid walls of the skull ; below is the interorbital septum ; 

 above, the lirm dorsal muscles and anterior interneural spines. 

 On the left or inner side are the coalesced limbs of the left 

 ectethmoid and sphenotic, separated from the parasphenoid by 

 the foramen already mentioned. Thus expansion is only possible 

 on the right or outer side, occupied by the eye, and on such part 

 of the left or inner side as is pierced by the foramen. 



The organ is rather richly suppHed with blood by a branch coming 

 from the vessel corresponding to the external carotid artery of 

 higher animals. This branch passes to the inner face of the 

 organ, whether left, or right, and breaks up into numerous smaller 

 vessels on that surface. 



I have already mentioned the lai'ge pad of adipose tissue 

 which underlies and more or less surrounds the left organ in the 

 HaHbut. It consists optically of a mass of connective tissue very 

 closely beset with minute oleaginous globules. The whole is 

 more or less elastic, and must add considerably to the contractility 

 of the organ it surrounds. To a less extent adipose matter is 

 found in the neighbourhood of the organ of the blind side in most 

 species which I have examined. 



I have studied the internal structure of the organ in various 

 species in the ordinary way, by means of microscopic sections, 

 but can find no trace of any glandular structure. The walls of 

 the organ and the interlacing muscular bands are merely lined 

 with ordinary flat epithelium cells. The liquid noticed as occurring 

 both in the organ and in the general orbital caA'ity is coagulated, 

 by the action of reagents, into a finely granular plasma, taking 

 on a faint pink stain in borax-carmine. It is indistinguishable 

 optically from the substance met with in similar preparations of 

 some of the brain-cavities of young fish. 



Professor Howes has drawn my attention to the fact that a 

 similar fluid is met with in mesenteric and synovial cavities, and, 

 in the absence of any definite secretory apparatus, is assumed 

 to be deposited there by the blood-vessels through the medium 

 of the ordinary epithelium cells. It seems permissible to draw 

 the same inference in the case of the organ now under discussion, 

 and to consider that the richness of the blood-supply is associated 

 with the production of the fluid. Similar fluid is present in the 

 orbital cavities of fish in which the recessus is not developed, but 

 of course m a less quantity. 



Function. 



We have seen that the cavity of the orbit and of the recessus 

 orlitaUs is filled during life with a fluid which has, without doubt, 

 the function of supporting the orbit, since the sinking of the eyes 

 which is always, unless averted by artificial means, to be observed 

 in stale fish, seems to be chiefly due to the coagulation and con- 

 sequent shrinkage of the fluid contents of the orbital cavities. 



It will be familiar to most observers that, whereas in a "round 



