18 bird's-eye yiews. 



cepted cliiiers from all of these hypotheses, and makes out 

 the mar&upium to be an "erectile" organ. Although no 

 muscular fibres have been shown to exist in it, yet it is 

 probably capable of expansion and contraction much as if it 

 were muscular. It is a highly vascular structure, as we 

 have seen ; and the increased or diminished turgidity of its 

 numerous blood-vessels* would, of course, alter its dimen- 

 sions. If it occupies a variable space in the vitreous humor, 

 it must alFect the position of the lens, and by this means 

 change the focus of the eye. This seems to be the most 

 satisfactoiy explanation, both of the design of the marsu- 

 pium, and of the mode in which its design is carried out. 

 In this view, the organ is marshalled with several others 

 that we know contribute to the greatest physiological phe- 

 nomenon of a bird's eye, — the rapid adjustment of focus. 



As anatomists, we have examined the structure, and posi- 

 tion, and appearance of the organs that make up a bird's eye. 

 But our study would be to little purpose if it ended here with 

 an inspection of dead tissues. We- have seen some curious 

 things that, perhaps, have afforded us gratification, which is 

 well enough as far as it goes ; but curiosity is only laudable 

 when, disdaining amusement as an ulterior object, it is con- 

 tented only with a higher aim, — instruction. We must 

 look, as physiologists, at the operations of the eye, and the 

 mode in which its functions are conducted and accomplished. 

 All that has gone before is merely to prepare us to question 

 intelligentl}' the structures we have examined, and find out 

 hoAV they work. 



Eyes are made to see with, of course : but how we see 

 with our eyes nobody knows. No one can tell us hoiv an 



*It is not apparent, at first sight, whence the marsiipium gets its numerous vessels, 

 since it is not attached at all to the vascular membrane of the eye, — the choroid. Pro- 

 fessor Owen remarks on this subject: "Branches of the ophthalmic artery, distinct 

 from the vessels of the choroid, and homologous with the arteria centralis retina, enter 

 the eye between the lamiuai of tlie retina, along the whole extent of the oblique slit 

 In the sclerotic and choroid), and immediately penetrate the folds of the marsupial 

 membrane, upon which they form <lelicate ramifications." (Anatomy of Vertebrates, 

 Vol. II, p. 1.39.) 



