TEE AUDITORY APPARATUS. 947 



Comparison of the Visual Apparatus of Man with that of Animals. 



Essential Organ of Vision.— The eyeball of Man is almost spherical, as in the 

 carnivora. 



The sclerotic does not differ much. The cornea has two limitary membranes, and is much 

 less elliptical than in Solipeds. The choroid has the same zone as in animals ; it is uniformly 

 brown. The ciliary processes, seventy to eighty in number, are a little longer than in the 

 Horse, and do not exceed, in front, the ciliary ligament, to the inner face of which they adhere 

 throughout their external border. The pupillary opening of the iris is always round. The 

 retina is the same in structure as already described. A little above the optic papilla, there is 

 a circular or oval patch, about 5', of an inch in diameter, in the centre of which is a transparent 

 spot ; this is the yellow spot (macula lutea), with the fossa centralis of the retina (fovea centralis, 

 foramen of Soemmering). 



At this patch, the tissue of the retina is slightly modified, especially at the fossa ; there are 

 only cones in the columnar layer, and all the other layers appear to be confounded into one 

 granular mass. (This spot only exists in animals which have the axes of the eyeballs parallel 

 with each other, as in Man, the Quadrumana, and some saurian Reptiles.) 



There is nothing particular in tlie aqueous humour, lens, or vitreous humour. 



Accessory Organs of the Visual Apparatus. — The orbital cavity in Man is entirely 

 enclosed by bony walls, and there is no fibrous sheath. (A fold of the orbital fascia has been 

 described as separating the eye from its surrounding adipose tissue, and which, like a "tunica 

 vaginalis," enables the globe to roll with rapidity and precision.) The muscles are six in 

 number — four recti, and two oblique; the great oblique is the same as in animals. Only the 

 rudiment of a caruncula lachrymalis is present. The nasal duct opens at some distance up on 

 the surface of the inferior meatus. 



CHAPTER V. 

 AUDITORY APPARATUS. 



The sense of hearing — destined for the perception of sounds produced by the 

 vibration of bodies — has for essential agents the auditory or eighth pair of 

 cranial nerves, the terminal fibrillae of which ramify in the membranous walls 

 of a system of cavities forming the internal ear. These cavities are excavated in 

 the substance of the petrous bone, and communicate, externally, by means of 

 two other systems of diverticuli, which constitute the middle and external ear. 



Article L — Internal Ear, or Labyrinth. 



The cavities which, together, compose this part of the auditory apparatus, 

 being entirely channeled within the petrous portion of the temporal bone, have 

 their walls — forming the osseous labyrinth — constituted by that bone. They 

 contain the soft parts, named the membranous labyrinth, and fluids {endolymph). 



The Osseous Labyrinth. 



This is composed of three portions : the vestibule, semicircular canals, and 

 cochlea. 



Preparation.— These cavities can be seen by making sections through the petrous bone, in 

 different directions. But it is better to expose them by cutting away this bone, after it has 

 beeu softened by prolonged steeping in dilute nitric acid. 



