912 THE ORGANS OF SENSE. 



ering of the lids or repeated twitchings, most commonly due to errors of refraction in children, 

 or more continuous spasm, due to some irritation of the fifth or seventh cranial nerve. The 

 Orbicularis may be paralyzed, generally associated with paralysis of the other facial muscles. 

 Under these circumstances the patient is unable to close the lids, and, if he attempts to do so, 

 rolls the eyeball upward under the upper lid. The tears overflow from displacement of the 

 lower lid, and the conjunctiva and cornea, being constantly exposed and the patient being unable 

 to wink, become irritated from dust and foreign bodies. In paralysis of the Levator palpebrae 

 superioris there is drooping of the upper eyelid and other symptoms of implication of the third 

 nerve. The eyelids may be the seat of bruises, wounds, or burns. In these latter injuries adhe- 

 sions of the margins of the lids to each other or adhesion of the lids to the globe may take 

 place. The eyelids are sometimes the seat of emphysema after fracture of some of the thin 

 bones forming the inner wall of the orbit. If shortly after such an injury the patient blows his 

 nose, air is forced from the nostril through the lacerated structures into the connective tissue of 

 the eyelids, which suddenly swell up and present the peculiar crackling characteristic of tin's 

 affection. 



The lachrymal gland is occasionally, though rarely, the seat of inflammation, either acute 

 or chronic ; it is also sometimes the seat of tumors, benign or malignant, and for these may 

 require removal. This may be done by an incision through the skin just below the eyebrow ; 

 and the gland, being invested with a special capsule of its own, may be isolated and removed 

 without opening the general cavity of the orbit. The canaliculi may be obstructed, either as a 

 congenital defect or by some foreign body, as an eyelash or a dacryolith, causing the tears to run 

 over the cheek. The canaliculi may also become occluded as the result of burns or injury ; over- 

 flow of the tears may in addition result from deviation of the puncta or from chronic inflamma- 

 tion of the lachrymal sac. This latter condition is set up by some obstruction to the nasal duct 

 frequently occurring in tubercular subjects. In consequence of this the tears and mucus accumu- 

 late in the lachrymal sac, distending it. Suppuration in the lachrymal sac is sometimes met 

 with ; this may be the sequel of a chronic inflammation ; or may occur after some of the erup- 

 tive fevers in cases where the lachrymal passages were previously quite healthy. It may lead to 

 lachrymal fistula. 



THE EAR. 



The organ of hearing is divisible into three parts the external ear, the middle 

 ear or tympanum, and the internal ear or labyrinth. 



The external ear consists of an expanded portion named pinna or auricle, and 

 the auditory canal, or meatus. The former serves to collect the vibrations of the air 

 by which sound is produced ; the latter conducts those vibrations to the tympanum. 



The pinna, or auricle (Fig. 539), is of an ovoid form, with its larger end directed 

 upward. Its outer surface is irregularly concave, directed slightly forward, and 

 presents numerous eminences and depressions which result from the foldings of its 

 nbro-cartilaginous element. To each of these names have been assigned. Thus 

 the external prominent rim of the auricle is called the helix. Another curved 

 prominence, parallel with and in front of the helix, is called the antihelix ; this 

 bifurcates above, so as to enclose a triangular depression, the fossa of the. antihelix. 

 The narrow curved depression between the helix and antihelix is called the fossa 

 of the helix (fossa innominata or scaphoidea) ; the antihelix describes a curve 

 round a deep, capacious cavity, the concha, which is partially divided into two 

 parts by the commencement of the helix. In front of the concha, and projecting 

 backward over the meatus, is a small pointed eminence, the tragns, so called 

 from its being generally covered on its under surface with a tuft of hair resem- 

 bling a goat's beard. Opposite the tragus, and separated from it by a deep notch 

 (incisura intertragica) is a small tubercle, the antitragus. Below this is the lobule, 

 composed of tough areolar and adipose tissue, wanting the firmness and elasticity 

 of the rest of the pinna. 



Structure of the Pinna. The pinna is composed of a thin plate of yellow fibro- 

 cartilage covered with integument, and connected to the surrounding parts by the 

 extrinsic ligaments and muscles, and to the commencement of the external 

 auditory canal. 



The integument is thin, closely adherent to the cartilage, and furnished with 

 sebaceous glands, which are most numerous in the concha and scaphoid fossa. 



The cartilage of the pinna consists of one single piece : it gives form to this 

 part of the ear, and upon its surface are found all the eminences and depressions 

 above described. It does not enter into the construction of all parts of the 

 auricle : thus it does not form a constituent part of the lobule ; it is deficient 



