SENSE OF SIGHT. 541 



movement of the tongue. It is a matter of common observation 

 that if sapid substances are simply placed on the tongue, the sense 

 of taste is not excited ; but if the tongue is pressed against the 

 roof of the mouth, absorption is promoted and the gustatory qual- 

 ities are at once recognized. 



It is to be noted also that a savor persists for a certain length 

 of time, and that if it is desired to determine the comparative 

 qualities of different substances by the sense of taste, there must 

 either be intervals between the tests or something must be used 

 to obliterate the taste of the first before the second is taken into 

 the mouth. It is also noteworthy that some savors so powerfully 

 impress the taste-organs that others subsequently fail to make any 

 impression. This principle is made practical use of in rendering 

 disagreeable medicines tasteless. Thus a few cloves eaten before 

 taking a dose of castor oil will render the latter far less nauseous ; 

 a mouthful of brandy will have the same effect. 



Sense of Sight. The eyes are situated in the orbits, cavities 

 formed by the frontal, sphenoid, ethmoid, superior maxillary, 

 malar, lacrimal, and palate bones. Each eye is embedded in fat 

 and enclosed in a serous sac, the capsule of Tenon or tunica 

 vaginalis oculi. The transverse diameter of the eye is about 2.5 

 cm., while the anteroposterior and vertical diameters are about 

 2.25 cm. each. 



Coats or Tunics (Fig. 320). These are 3 in number: (1) Scle- 

 rotic and cornea ; (2) choroid, ciliary processes, iris, and ciliary 

 muscle ; and (3) retina. 



Sclerotic. This tunic forms the external coat of the eye for its 

 posterior five-sixths, the anterior sixth being formed by the cornea. 

 The sclerotic is opaque, and is made up of dense fibrous tissue 

 with elastic fibers and connective-tissue corpuscles, and the white 

 color of its visible external surface covered by conjunctiva causes 

 this portion to be known as the white of the eye. The internal 

 surface is lined by connective tissue, in which are brown pigment- 

 cells, lamina fusca. This inner surface is grooved, and in these 

 grooves lie the ciliary nerves. Posteriorly and on the nasal side 

 the optic nerve pierces the sclerotic, which in this part is charac- 

 terized by small openings, through which the fibers composing the 

 optic nerve pass. This sieve-like structure has given to the scle- 

 rotic at this point the name lamina cribrosa. The largest of these 

 openings, porus opticus, is in the middle of the lamina, and 

 through it passes the arteria centralis retince (Fig. 328, A). En- 

 circling the lamina cribrosa are openings in the sclerotic through 

 which pass the ciliary vessels and nerves. Supplying the sclerotic 

 itself, nerves have not been demonstrated. 



Cornea. This structure is transparent, and its relation to the 

 sclerotic has been compared to that of a watch-crystal to the 



