944 THE SENSORY ArPARATUSES. 



which is insinuated between all the muscles of the eye, and to which it is loosely 

 attached. No muscle directly concurs in the movements of this body : they are 

 entirely mechanical. When the eye is in its usual position, there is only per- 

 ceived the fold of conjunctiva that terminates it in front ; the remainder is 

 concealed in the fibrous case of the eye. When, however, the latter is with- 

 draw-n into the orbit by the contraction of its recti muscles, the globe compresses 

 the fatty cushion belonging to the cartilage ; this cushion, pressing outwards, 

 pushes the membrana before it, and the latter then entirely conceals the whole 

 front of the globe. This movement is instantaneous, but it may be momentarily 

 checked by pressing gently on the eye when the animal retracts it within the orbital 

 cavity. 



The use of the membrana is— as will be seen from the above — to maintain the 

 healthy condition of the eye, by removing any matters that have escaped the 

 eyelids ; and what clearly demonstrates this function, is the inverse relation that 

 always exists between the development of this body, and the facility with which 

 animals can rub their eyes with their anterior limbs. So it is that, with the 

 Horse and Ox, the thoracic limb of which cannot be applied to this purpose, 

 the membrana is very developed ; in the Dog, which may use its paw to some 

 extent when it requires to brush its eye, it is smaller ; in the Cat it is still less ; 

 while in the Monkey and in Mankind, whose hands are perfect, it is rudimentary. 

 In Tetanus, the membrana nictitans often remains permanently over the eye, in 

 consequence of the continued contraction of the recti muscles. 



(Towards the middle of the outer face of the membrana is a small yellowish- 

 red, acinous gland — the glandula Harden, firmly boimd by a strong fibrous 

 membrane to the cartilage, and surrounded by adipose tissue ; it secretes a thick 

 unctuous matter, which escapes by two or three small apertures on the inner face 

 of the membrana.) 



LACHRYMAL APPARATUS. 



Preparation. — The lachrymal gland is prepared at the Bame time h8 the levator palpebrarum 

 (see above). To disBect its excretory apparatus, melti'd tallow should be injected into tl;e 

 lachrymal canals by the nasal opening of the duct. The lachrymal bone should be chiselled 

 away, in order to see the canal ; the nasal portion can be shown in a longitudinal and vertical 

 section of tlie head, made outside the median plane. 



This apparatus comprises : 1 . A gland which secretes the tears. 2. A series 

 of canals that carry the superfluous fluid to the external orifice of the nasal 

 cavities. 



Lachrymal gland. — This gland, situated between the orbital process and the 

 upper part of the eyeball — from which it is separated by the superior rectus and 

 levator palpebrae muscles — is convex on its upper face, and concave inferiorly, in 

 accordance with the parts it adjoins. Only little developed, it is formed of very 

 small granules, united by fine connective tissue ; from these arise minute radicles, 

 the junction of which forms a certain number of very narrow ducts, that open 

 on the inner face of the temporal (outer) angle of the eyelids. These are the 

 hygrophthalmic canals. 



The lachrymal gland secretes the tears that lubrify the anterior surface of 

 the eye. This fluid escapes upon the organ at the temporal angle of the lids, 

 and is carried between them and the eyeball towards the nasal angle. Its 

 secretion is incessant, but it is increased by anything that irritates the con- 

 junctiva, and its character may even change under the same influences. 



