670 PHILOSOPHICAL TRANSACTIONS. [aNNO ]7q6. 



the marsupium measured — of an inch, and the folds were much more opaque. 

 Here then was a difference of -5^ of an inch in the length of the two marsupia; 

 which could arise from no other cause than the one having contracted so much 

 more than the other, which contraction we must consider as muscular. Haller 

 denies the marsupium to be muscular, because there is no such appearance in its 

 structure. My own opinions on the structure of muscles have been already 

 explained to this learned Society ; and I have lately met with an observation in 

 Lyonet's dissection of a caterpillar which tends to confirm them. He says, the 

 muscles of the caterpillar are, in their natural state, transparent as geily, and 

 have vessels passing through their substance in every direction, which afford to 

 the eye of the observer in the microscope the most beautiful appearance of a 

 congeries of vessels.* 



The peculiarities in the bird's eye are such as tend to facilitate both the length- 

 enino- the axis of vision, and increasing the convexity of the cornea. The bonv 

 rim, to which the muscles are attached, confines the effect of their pressure to 

 the broadest part of the eye; and as their action throws forwards the cornea, the 

 anterior edge of the bony rim yields, to adapt itself to that change; the ciliary 

 processes are long, to admit of the lens being moved forwards, and by their 

 action bring it back, to its place; by these means the eyes of birds are adjusted 

 to see very near objects with more facility than the eyes of other animals. As 

 the eyes of birds are also to be adjusted to see very distant objects, the marsu- 

 pium is placed behind the crystalline lens, to draw it backwards, and when it 

 acts, part of the pressure from behind being removed, the cornea is rendered 

 flatter; and the anterior edge of the bony rim is adapted to it, in this state, by 

 the contraction of the annular elastic ligament. It may be said, that to see with 

 parallel rays no such great change is necessary; it must however be considered, 

 that where vision is to be very distinct, a certain nicety of adjustment becomes 

 necessary, and the action of the marsupium is probably intended for that purpose. 



In the bird, though not immediately connected with the present subject, 

 there is one of the most beautiful illustrations of the combination of muscular 

 and elastic substances. This is employed for the motion of the membrana nic- 

 titans, and as it shows tliat such a combination is adopted wherever it can be 

 used with advantage, and is provided as a defence for the organ in which I am 

 endeavouring to explain such a combination, I cannot avoid taking notice of it. 

 The membrana nictitans is composed of an elastic membrane, which is con- 

 nected by means of a tendon, with 2 muscles situated on the posterior part of 

 the eye-ball; the action of these muscles brings the membrane over the cornea, 

 and the instant they cease to contract, the elasticity of tiie membrane draws it 

 back, again. 



* Traite Anatomique de la Chenille, par Pierre Lyoiiet, chap. <), page i) -'. — Orig. 



