AVES. 



307 



semilunar tendinous sheath, having no express 

 or fixed insertion. The second muscle, called 

 Pyntmiddlis nictitantis, (h, jig. 139,) arises 

 from the sclerotica from the lower and 

 nasal side of the eye-ball ; its fibres con- 

 verge as they pass to the upper side of the 

 optic nerve, and there terminate in a small 

 round tendon, which glides through the pulley 

 at the free margin of the quadratus, and wind- 

 ing round the optic nerve, passes along a cellu- 

 lar sheath at the lower part of the sclerotica, 

 and is inserted into the lower part of the mar- 

 gin of the third eye-lid, along which it is 

 continued for some distance, and is gradually 

 lost. 



By the simultaneous action of the two mus- 

 cles, the membrana nictitans is drawn forcibly 

 outwards and with an oblique inclination down- 

 wards over the anterior part of the eye.* The 

 tendon of the pyramidalis gains the due direc- 

 tion for that action by winding round the optic 

 nerve, and it is restrained from pressing upon 

 that nerve during the action of the pyramidalis 

 muscle by the counteracting force of the qua- 

 dratics, which thus augments the power of the 

 antagonist muscle, while it obviates any incon- 

 venience from pressure on the optic nerve, 

 which its peculiar disposition in relation to that 

 part would otherwise occasion. 



To examine this singular and beautiful me- 

 chanism, it is necessary to remove the muscles 

 of the eye-ball, especially the recti. 



Lachrymal Organs. There are two glands 

 .which secrete a fluid to lubricate the ball of 

 the eye, and facilitate the movements of the 

 eye-lids ; one of these relates more especially to 

 the movements of the nictitating membrane, 

 and is called from its discoverer the Hardcrian 

 Gland ; the other corresponds to the ordinary 

 Glandula lachrymalia. 



Fig. 140. 



This oblique motion is most remarkable in the 

 Owls, in which the nictitating membrane is ac- 

 companied by the- upper eye -lid in its sweeping 

 movement across the eye-ball. 



The Glandula Hurdcriana (i 1 Jig. 140) 

 is a conglomeration of mucous follicles, which 

 compensates for the absence of Meibomian 

 glands in Birds; it is generally of large size, 

 situated at the internal angle of the eye, 

 and pours out a thick viscid secretion by a 

 small duct which opens beneath the nictitating 

 membrane. The surface of the gland is di^ 

 vided into many small lobules, which, when 

 injected with mercury, are seen to be com- 

 posed of still smallervesicles. 



It is interesting to find that some of the 

 Rodentia, which manifest so many affinities 

 to the Class of Birds, have a corresponding- 

 gland ; in the Hare, for example, it is of large 

 size and bipartite, situated at the internal angle 

 of the orbit, and opening beneath the internal 

 eye-lid. 



The true lachrymal gland is situated at the 

 external angle of the eye. In the Goose it is 

 of a flattened form, about the size of a pea, 

 opening upon the inside of the outer angle of 

 the eye-lids by a short and wide duct. Its 

 secretion is less viscid than that of the Har- 

 derian gland : but this is not uniformly the 

 case. 



The lachrymal duct consists of a wide mem- 

 branous canal commencing by two apertures at 

 the nasal canthus of the eye, and terminating 

 below and a little before the middle or great 

 turbinated cartilage. In the Ostrich there is 

 a glandular prominence at the commencement 

 of each of the lachrymal canals ; these seem 

 analogous to the caruncula lachrymalis. In 

 other birds this structure is wanting. 



Nasal gland, (k, Jig. 140.) Besides the 

 lachrymal glands, or those which furnish a 

 fluid for the purpose of lubricating, defending, 

 and facilitating the movements of the eye-ball, 

 there exists another gland, which, from its 

 position within or near the orbit, seems at first 

 sight to appertain to the preceding series, but 

 the secretion of which is exclusively employed 

 in lubricating the pituitary membrane of the 

 nose. This gland, which corresponds to the 

 nasal glands of serpents, and those described 

 by Jacobson* in Mammalia, is situated in 

 many aquatic and marsh birds above the 

 supra-orbital ridge in a depression noticed in 

 the description of the skull, (p. 278.) In 

 most birds it is lodged within the orbit itself; 

 in some it is found under the nasal bone, or 

 in the cavity analogous to the maxillary sinus. 

 In the Woodpeckers it is found in the sub- 

 ocular air-cell. It appears to be present in 

 every order of Aves.f 



In the Anserine Birds this gland is so situ- 

 ated as to complete the superior margin of the 

 orbit, (k r , Jig. 140,) and is inclosed in an ex- 

 tremely dense fibrous membrane. Its duct 

 (/, fig. 140) is long, and passes to the nose 

 along an osseous groove, behind the lachrymal 

 bone. Its structure is simple, like that of the 

 salivary glands in the same class, being com- 

 posed of ramified follicles from which the 



* Nouv. Bullet, des Sc, par la Soc. Philomath, 

 iii. an 6. p. 267. 



t Nitxsch, Meckel's Archiv. vi. p. 234. 



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