CHAP. XIX.] ORGANS OF SPECIAL SENSE. 227 



the impression of a continuous sound ; and this sound has a 

 higher or lower pitch, according to the rapidity of its vibra- 

 tions. It has been discovered that sonorous impulses following 

 each other with a rapidity of less than sixteen times per second, 

 are separately distinguishable ; but above that frequency they 

 are merged into a continuous sensation. When the sonorous 

 impulses are repeated at irregular intervals, the only characters 

 perceptible in the sound are its intensit} r and quality. But if 

 they succeed each other at regular intervals, the sound pro- 

 duced has a position in the musical scale as a high or low note. 

 The more frequent the repetitions, the higher the note ; but a 

 limit is at last reached, at which the ear fails to perceive the 

 sound, and an excessively high note is therefore inaudible. 

 Sonorous vibrations, perceptible to man as musical notes, range 

 between sixteen per second for the lowest notes, and 38,000 for 

 the highest. (Dalton.) 



The sense of sight. The eye is the special organ of the sense 

 of sight, and consists of the eyeball, or eye proper, and of acces- 

 sory protective appendages, such as the eyebrows, eyelids, lach- 

 rymal glands, etc. 



The eyeball is contained in a bony cavity, the orbit, which is 

 padded with fat and lined with a membranous capsule, the 

 capsule of Tenon. This capsule is a serous sac, one layer of 

 which is attached to the posterior portion of the eyeball, while 

 the other lines the orbital cavity: in this way the eyeball is 

 isolated from surrounding structures, and free movement with- 

 out friction is insured. The orbit is shaped like a four-sided 

 pyramid ; the apex, directed backwards and inwards, is pierced 

 by a large opening, the optic foramen, through which pass 

 the nerves and blood-vessels distributed to the eyeball. The 

 base of the orbit, directed outwards and forwards, forms a strong 

 bony edge for protecting the eyeball from injury. 



The eyeball is spherical in shape, but its transverse diameter 

 is less than the antero-posterior, so that it projects anteriorly, 

 and looks as if a section of a smaller sphere had been engrafted 

 on the front of it. 



The eyeball is composed of three coats or tunics, and contains 

 three refracting media or humours. They are as follows : 



Tunics. 1. Sclerotic and cornea. 



2. Choroid, iris, and ciliary processes. 



3. Retina. 



