1889.] MICROSCOPICAL JOURNAL. 147 



Elementary Histological Studies of the Cray-fish — XIII. 



By HENRY LESLIE OSBORN, 



HAMLINE, MINN. 



chapter v. — the eye. — {Concluded.) 



4. The Stalk. 



1. The Epidermis. — A little in front of the basilar membrane a 

 great thickening of the epidermis may be seen suddenly to take place. 

 Here the ordinary calcareous shelly matter of the general surface com- 

 mences and it forms the coating of the stalk. It, with the cornea, is 

 shed when the peculiar process of moulting or shedding takes place 

 and a new epidermis takes its place, secreted from the layer next be- 

 neath. 



2. The Hypodermis. — This is the living tissue of the skin, that from 

 which the epidermis is produced. It presents no features which re- 

 quire our attention. 



3. The Optic Nerve. — The centre of the stalk is occupied by two 

 kinds of tissue — connective tissue, an inert body used here and every- 

 where as a skeleton or framework to support the delicate active tissue, 

 the nerve-bundle. This in the lower portion of the stalk is composed 

 of fine parallel fibres, the nerve-fibres, extremely fine bodies traversing 

 the body from the anterior chamber of the eye to the brain. These 

 threads are too delicate and easily injured to show very well in prep- 

 arations. 



4. The ganglion cells. — Along the nerve trunk, at various places, 

 and particularly" the border, pear-shaped bodies may be discovered. If 

 these are carefully examined under a high power they are seen to be 

 very granular and well stained and to contain a round body or nucleus, 

 less stained but containing numerous very deeply stained particles. 

 These masses of cells may show cell outlines only indistinctly or may 

 show them more clearly as in Fig. 3. (Plate in February number.) 

 In Fig. 2 one of these cells is shown and bits of nerve fibres. 



5. Near the basilar membrane nerve-fibre matter, as at 1 and 4, seems 

 to be interrupted by finely granular matter not arranged as through 

 most of the length of the nerve. 



6. Muscles. — Outside the optic nerve, between it and the hypo- 

 dermis, may be seen ribbands of striped muscular tissue. 



5. The interpretation of the observations. — The physiologist 

 is always upon dangerous ground when he undertakes to interpret 

 from the appearances of sections to the anatomy of organs and then to 

 their mode of working. Some of the facts in the structure of the eye 

 we can readily reconstruct from our sections ; the cylindrical stalk and 

 hemispherical anterior chamber, the central nerve and the muscles, the 

 uses of these parts to the muscles for producing the motions of the eye, 

 which we notice so quickly on observing the living creature. But the 

 nervous parts are more puzzling. The basilar membrane is probably 

 a connective tissue substance of somewhat more compact nature than 

 that below, to help keep in place the rods ; the ganglion cells of the 

 nerve are connected with the work of transmitting nerve impulses when 

 once they are started in the nerves. 



The diagram, Fig. 5, gives from Patten* a view of the eye as under-, 



* American Journal of Morphology, vol. i, p. 92. 



