332 PHILOSOPHICAL TRANSACTIONS. [ANNO 1798. 



the foramen lacerum orbitale inferius. In the bullock and sheep there is a plexus 

 of vessels surrounding the optic nerve, and the tube dips down, close by the optic 

 nerve, probably to accompany them. 



From the observations made by Michaelis, of the yellow spot not being visible 

 in foetuses, or in infants under a year old, or in eyes that are blind, also of its being 

 brighter in young people, and paler in old, it would appear, that it is only when 

 the eye is capable of performing its functions, that there is any stain communicated 

 to the retina. The drawings from which the figures are engraved, were made from 

 preparations of the eye lying in water, with a strong light shining on the prepara- 

 tion. In making the drawings, the principal object was, procuring a distinct view 

 of the parts surrounding the optic nerve; when this could be obtained, the situa- 

 tion of the eye itself was not attended to. 



Fig. 5, pi. •*>, is a transverse section of the human eye, immediately before the ciliary processes. 

 The retina is viewed through the posterior portion of the capsule of die crystalline lens, a, The ter- 

 mination of the optic nerve, b, The aperture in the retina, discovered by Professor Soemmering. 



Fig. 6, A longitudinal section of the left eye in the human subject, to show the relative situation of 

 the aperture in the retina to the entrance of the optic nerve, and the mode in which it appears to pro- 

 ject, when the vitreous humour is disturbed, a, The termination of the optic nerve, b, The aperture 

 in the retina. 



Fig. 7, A transverse section of the eye of a large monkey, to show the aperture in the retina : its 

 situation is the same as in the human eye. The zone has the appearance of a star with 4 rays, a, The 

 entrance of the optic nerve, b, The aperture in the retina. 



Fig. 8, A transverse section of the eye of a bullock, to show that there is a semi-transparent tube 

 projecting from the edge of the entrance of the optic nerve, into the vitreous humour. This tube is 

 surrounded by a zone, with a distinct margin : it is situated on the temporal side of the optic nerve. 



Fig. 9, A transverse section of the eye of a sheep, to show that there is a similar tube as in the bul- 

 lock, in the same situation, but much shorter, and without the surrounding zone. 



XIII. On a very Unusual Formation of the Human Heart. By Mr. J. Wilson, 



Surgeon, p. 346. 



It is well known that the circulation of the blood throughout the body, and 

 exposure of it to the atmospheric air in respiration, seem in most animals to be 

 necessarily connected; but are not equally so in all. They are so much connected 

 in the human subject, and in most quadrupeds, that after birth there is a double 

 heart; viz. one for the circulation of the blood throughout the body, to be sub- 

 servient to the various purposes of life and growth; the other for its circulation 

 through the lungs, where it undergoes a change which is essential to its general 

 circulation through the body : these 2 circulations, in the natural state, bear an 

 exact proportion to each other. Instances however have occurred, even in the 

 human subject, where this exact proportion has not been preserved; yet life has 

 been prolonged for some years, but in a feeble and imperfect state. In some of 

 these instances, the pulmonary artery has been smaller than usual, so that much 

 less than the natural quantity of blood was exposed to the influence of the air in 

 the lungs ; in others the foramen ovale has not been closed, but a considerable 



