Book - The Embryology Anatomy and Histology of the Eye
In approaching this work, perhaps a word of explanation to the reader may be desirable. It is not undertaken because the author thinks there is a lack of knowledge about the eye; neither have there been any new facts discovered which would merit the production of these articles. It is therefore not the intention to bring out any new facts, but to put the known and widely scattered facts in a more comprehensible form and to illustrate the subject so thoroughly and completely that it will be made more easy for the beginner and more interesting to those who find it necessary to review the subject.
All the illustrations of the structures of the eyeball and smaller structures will be microphotographs taken from microscopic slides in the author's possession, while the coarser structures of the orbit will be illustrated by drawings, as these structures are too large for the tissues to be mounted on microscopic slides.
The microscopic slides used to photograph the foetal eye are from the pig and were procured at the Armour packing house by collecting the foetal pigs at the gutting table. These foetesis ran from two millimeters to forty millimeters in length, and the mounting of the slides was done by Dr. Slonaker, at the Chicago University.
The slides used in photographing the adult eye were made by Dr. Slonaker when he wrote his thesis on the acute area of vision. These slides have been used in my illustrated lectures before optical and medical societies for several years, and they have been enjoyed so much by my hearers and I have received so many requests for them in a i)ermanent form, that it is in response to these wishes that the author lias determined to perpetuate these pictures and place them in the reach of every one who is interested in the eye ; otherwise these articles would never have appeared. A few words about the physical development of the foetus might be of benefit before the illustrations are studied. The foetus is first represented by one cell, the ovum. This is fertilized by the spermatozoa; then there is a multiplication of cells. These increase very rapidly, and the first definite form assumed is a tube, representing the worm, and this tube has two walls ; one is the outer covering and the other lines the inside. The outer is known as the cpiblast (meaning above) and the inner the hypoblast (meaning below). Then there is a layer developed between these two layers. This layer is known as the mesablast (meaning the middle). From the epiblast is developed the skin and nervous system. From the hypoblast is developed the alimentary canal and all the internal organs which communicate with the alimentary canal. From the middle layer, or mesoblast, is developed the connective tissue, blood vessels, muscles, bones, etc.
From the foregoing we see that in the study of the eye we are most especially concerned in the epiblast, as it forms the nervous system and therefore the brain, and the inner seat or sensory coat of the eye, and some one has well said that the eye is a part of the brain placed near the surface, back of an opening, where it may receive impressions from the external world and communicate these impressions to the main portion of the brain.
The first indication of the nervous system commences by the development of two ridges along the dorsum, or back, of the foetus during its tubular development. These are known as the neural ridge$. The cells composing these ridges multiply and they rise higher and higher and finally meet al)Ovc, at the center, and coalesce, or grow together, leaving an opening. Tliis is knowm as the neural