626 THE AQUEOUS HUMOUR. 



AQUEOUS HUMOUR ANT> ITS CHAMBER. 



The aqueous humour fills the space in the fore part of the eyeballj 

 between the capsule of the lens -with its suspensory lio;ament and the 

 cornea. It differs little from water in its physical characters ; but it 

 contains a small quantity of some solid matter, chiefly chloride of 

 sodium, dissolved in it. The iris, resting in part upon the lens, divides 

 the aqueous chamber partially into two, named respectively the anterior 

 and posterior chambers. This subdivision is incomplete in the adult, 

 but in the foetus before the seventh month it is completed by the mem- 

 brana pupillaris, which, by its union witli the margin of the pupil, 

 closes the aperture of communication between the two chambers. 



The anterior cMmher is limited in front by the cornea and behind 

 by the iris, while opposite the pupil it is bounded by the front of the 

 lens and capsule. 



The posterior chamber was originally so named in the belief that a 

 distinct free space intervened between the iris and the capsule of the 

 lens. It is now, however, well ascertained by observations on the living 

 eye, and by sections made in the frozen state, that the iris comes into 

 contact with the capsule of the lens, both at the pupillary margin and 

 at the adjoining part of the posterior surface ; and the term posterior 

 chamber can therefore — unless it be employed to indicate the want of 

 lontinuity between those opposed structures, where no space actually 

 intervenes— only be applied to the angular interval existing at the 

 circumference between the ciliary processes, the iris, and the suspensory 

 ligament.* 



THE EAK. 



The organ of hearing is divisible into three parts : the external 

 ear (fig. 440, 1, 2), the tympanum or middle ear (.3), and the labyrinth 

 or internal ear (6). The first two of these are to be considered as 

 accessories to the tliird, which is the sentient portion of the organ. 



THE EXTERNAL EAR. 

 In the external ear are included the pinna — the part of the outer ear 

 which projects from the side of the head — together with the meatus or 

 passage which leads thence to the tympanum, and which is closed at 

 its inner extremity by a membrane (membrana tympani) interposed 

 between it and the middle ear. 



THE PINNA. 



Superficial configuration. — The general form of the pinna or 

 auricle, as seen from the outside, is concave, to fit it for collecting and 

 concentrating the undulations of sound, but it is thrown into various 

 elevations and hollows, to which distinct names have been given. The 

 largest and deepest concavity, a little below the centre of the organ, is 

 called the concha (fig. 441, 7) ; it surrounds the entrance to the external 

 auditory meatus, and is unequally divided at its upper and anterior 

 part by a ridge, which is the beginning of the helix. In front of the 



* For the most recent and complete account of the anatomy of all the parts composing 

 and counected -vvith the organ of vision the reader is refeiTed to the elaborate articles by 

 Merkel, "Waldcyer, Schwalbe, I-wanoff, and Arnold, in Graefe and Saemisch's Handbuch 

 der gesammten Augenheilkunde ; which, we may add, has been freely consulted in com- 

 piling the present description. 



