THE ORGANS OF SIGHT. 415 



been described as an olfactory organ. (Rosenthal.) This organ presents 

 great varieties among the decapoda ; in the spiny crab its covering is crusta- 

 ceous, and little muscular bundles are found beneath it. In the squilla it is 

 altogether wanting ; but in them, in certain species, there is found in the base 

 of the second and seventh thoracic pairs of feet, and in mysis, in the inner 



G" P of the tail-plates, a completely closed sac, containing a spherical crystalline 

 y, provided with stiff bristles, which has been regarded as an auditory 

 organ, analogous in its formation with the simple auditory vesicles of the 

 lower Mollusca. In the Annelida or worms, a pair of ciliated auditory vesi- 

 cles, with contained otoliths, is often present in the head ; they are connected 

 with the oesophageal ring. The great variability in the seat of the auditory 

 apparatus is accordingly quite as marked in the Annulosa as in the Mollusca, 

 as is exemplified especially in the grasshopper, locust, mysis, and squilla, in 

 which, as in the Gasteropodous and Lamellibranchiate Mollusca, it is asso- 

 ciated with the pedal or locomotive ganglia, or even with some part of the 

 locomotive apparatus. In the Vertebrata, likewise, the organs of hearing are 

 connected with the back of the medulla oblongata, lower than the centres of 

 origin of the nerves of the other special senses, and nearer, therefore, to the 

 motor apparatus generally. 



Amongst certain of the Annuloida, as in the marine Turbellaria, and per- 

 haps also in some Rotifera, an auditory vesicle, containing an otolith, and no 

 longer double and symmetrical, but single, is found lying closely on the chief 

 nervous ganglion. But most of the Rotifera, and all the Entozoa, are destitute 

 of special auditory organs ; nor has any such apparatus been detected in the 

 Echinodermata. 



In the Coelenterata, however, there are found, in both the discoid and cteno- 

 phorous forms, but chiefly in the medusae, auditory sacs named lythocysts, which 

 inclose crystalline particles, supposed to be analogous to otoliths ; these are 

 numerous, and are found on the margin of the disc ; they frequently have pig- 

 ment spots, or ocular spots, near them. 



The Protozoa are entirely destitute of auditory organs. 



THE SENSE OF SIGHT. 



The Organs of Sight. 



The organs of sight, in Man, consist of the eyeballs or globes of the 

 eyes. The external protective appendages of the eye, are the eyebrows, 

 the eyelids, and the lachrymal gland arid apparatus. The eyeballs 

 and the lachrymal glands, are lodged in the bony cavities, named the 

 or bits. 



The orbits are pyramidal in shape ; their apices are directed back- 

 wards and inwards, so that their axes converge posteriorly, arid diverge 

 anteriorly. In the apex of each orbit, are several openings, which 

 transmit the optic nerve, the common sensory, motor, and sympathetic, 

 nerves of the eyeball, as well as its bloodvessels and lymphatics. The 

 orbit also contains the lachrymal gland, the ocular muscles, and a 

 quantity of fat, on which the eyeball rests and moves as upon a soft 

 cushion ; between this and the eyeball, is a loose cellular capsule. 



The eyebrows, or the arched eminences surmounting the orbits, con- 

 sist of thick musculo-cutaneous ridges, inclosing some fat, and studded 

 with hairs set obliquely outwards. 



The eyelids, or palpebra?, are the two thin movable covers of the 

 eyeball, the free margins of which are bevelled, and beset with the 

 eyelashes. The upper one is larger and more movable than the lower 

 one, and is provided with a special muscle, named the levaior palpe- 



