1480 



HUMAN ANATOMY. 



which is usually selected, as there is less danger of laceration of the lining mucous 

 membrane leading to narrowing or occlusion of the canaliculus by scar tissue. 



Co7igenital JistulcE sometimes result from non-closure of the groove from which 

 the sac and nasal duct are formed. The lachrymal sac is situated at the inner side 

 of the inner canthus, behind the inner palpebral ligament, which is the best guide to 

 it, and crosses about the junction of the upper and middle thirds of the sac, 



A collection of mucus or pus in the lachrymal passage is usually in the sac, and 

 when not otherwise relieved, it tends to discharge itself through the skin below 

 the -tendo oculi, and frequently Jower than the level of the sac. The abscess is 

 therefore opened below the tendon and external to the inner edge of the lachrymal 

 groove. 



The line of the sac and duct, taken together, is approximately from the inner 

 canthus to the space between the second premolar and first molar teeth. It opens 

 below into the inferior meatus of the nose, just below and behind the anterior end of 

 the inferior turbinate bone, which conceals it from view at the anterior naris. The 

 sac and duct form a slightly curved line with its convexity backward, and its course 

 downward, backward and slightly outward. To pass a probe along the lachrymal 

 passage, the lower lid is everted by the thumb so that the punctum may be seen. 

 The probe should be entered into the punctum vertically. It should then be turned 

 horizontally and passed through the canaliculus to the inner wall of the lachrymal 

 sac. It is then made vertical and passed along the duct — i.e.^ downward, slightly 

 backward, and outward to the nose. 



DEVELOPMENT OF THE EYE. 



The development of the eye begins as a lateral diverticulum which very early appears on 

 either side of the fore-brain (Fig. 911). These outgrowths, the primary optic vesicles, are 

 hollow and directly communicate with the general cavity of the primitive brain by means of 

 the optic stalks, which are at first broad, but later become narrowed. As the development 

 proceeds, the transversely placed optic stalks gradually assume a more oblique axis, and, after 

 the differentiation of the fore-brain into its two subdivisions, open into the diencephalon or 

 inter-brain. The primary optic vesicle expands until it comes into contact with the surface 

 ectoblast. The next important step is a thickening of the wall of the vesicle where it is in con- 

 tact with the ectoblast (Fig. 1238). In consequence of the rapid multiplication of its cells, 

 this portion of the wall becomes invaginated and, as a result, the cavity of the primary optic 



vesicle is gradually obliterated, the application ot 

 Fig. 1237. the invaginated portion of the wall to the inner sur- 



face of the uninvaginated part of the vesicle bringing 

 about the formation of a cup-shaped structure pro- 

 vided with a double wall. This cup is called the 

 secondary optic vesicle and from it the retina is 

 developed, which must be considered, therefore, as a 

 modified portion of the brain itself. 



Coincidentally with the invagination of the optic 

 vesicle, the overlying ectoblast undergoes active 

 proliferation and pushes into the space vacated by 

 the receding invaginated wall, thus producing a 

 depression known as the lens-pit. The lens-pit (Fig. 

 1238) deepens and becomes cup-shaped ; the edges 

 of its anterior walls approach each other and then 

 fuse, and in this manner form a closed sac, the lens- 

 vesicle. This remains for a time connected with the 

 surface ectoblast, but later becomes separated from 

 it and forms an isolated sac of epidermal tissue, which, 

 by the proliferation of its cells, becomes converted into a solid structure and constitutes the 

 crystalline lens. At first the lens-vesicle fills the cavity of the optic cup completely, but with 

 the deepening of the latter, a space appears between itsranterior wall and the lens-vesicle, 

 which gradually widens and forms the vitreous cavity. The space between the lens-vesicle 

 and the ectoblast is invaded by a process from the surrounding mesoblast, which pushes in 

 from the side. From this ingrowth is developed the cornea, with the exception of the surface 

 epithelium, and the stroma of the iris. 



Almost from the first appearance of the invagination of the primary optic vesicle, the 

 invaginated portion of the wall exhibits a marked tendency to proliferation of its cells. The 



Optic 

 vesicle 



Part of frontal section of head of early rabbit 

 embryo, showing optic vesicles evaginated from 

 brain-vesicle. X 30. 



