266 GENERAL ORNITHOLOGY part ii 



covjunciiva. The pouch of the conjunctiva very generally communi- 

 cates, by the lacryriial dud, with the cavity of the nose. It may be 

 raised, on its inner side, into a broad fold, the nictitating membrane, 

 moved by a proper muscle or muscles. Special glands — the lacrymal 

 externally, and the Harderian on the inner side of the eyeball — may 

 be developed in connection with, and pour their secretion on to, the 

 conjunctival mucous membrane. The posterior chamber of the eye 

 has a totally distinct origin. Very early that part of the anterior 

 cerebral vesicle, which eventually becomes the vesicle of the third 

 ventricle, throws out a diverticulum, broad at its outer, narrow at 

 its inner end, which applies itself to the base of the tegumentary 

 sac. The posterior, or outer, wall of the diverticulum then becomes, 

 as it were, thrust in, and forced towards the opposite wall by an 

 ingrowth of the adjacent connective tissue ; so that the primitive 

 cavity of the diverticulum, which, of course, communicates freely 

 with that of the anterior cerebral vesicle, is obliterated. The broad 

 end of the diverticulum acquiring a spheroidal shape, while its 

 pedicle narrows and elongates, the latter becomes the optic nerve, 

 while the former^ surrounding itself with a strong fibrous sclerotic 

 coat, remains as the posterior chamber of the eye. The double 

 envelope, resulting from the folding of the wall of the cerebral optic 

 vesicle upon itself, gives rise to the retiiia and the choroid coat, the 

 plug or ingrowth of connective tissue gelatinises and passes into the 

 vitreous humotir, the cleft by which it entered becoming obliterated." 

 (Anaf. Vert., 1871, p. 79.) 



Birds alone, of all animate beings, may be truly said to " fall 

 asleep " in death. When the " silver cord " of a bird's life is loosed, 

 the " windows of the soul " are gently closed by unseen hands, that 

 the mysterious rites of divorce of spirit from matter may not be 

 profaned. When man or any mammal expires, the eyes remain 

 Avide open and their stony stare is the sign of dissolution. Only 

 birds close their eyes in dying. At the same moment, the eye 

 sinks and seems to collapse, by the ebbing of its waters. The 

 closure is chiefly effected by the uprising of the lower lid. These 

 are the principal external differences between the eyes of birds and 

 mammals. The movements of the upper lid in most birds are much 

 more restricted than those of the lower. The few exceptions are 

 chiefly furnished by night birds, as owls, whip-poor-wills, and others 

 of their respective tribes. The lids consist externally of common 

 skin, internally of a layer of conjunctival (joining) mucous membrane, 

 with interposed connective tissue : the lower is also stiff'ened with 

 a smooth plate, the farscd cartilage. The upper is raised by a small 

 muscle, called from its office levator pcd])cbrce sxipcrioris, arising from 

 the bony orbit. There is no special lowering nor lifting muscle of 

 the under lid ; the lids close together by the action of the orbicularis 



