ANIMAL MECHANISM. 103 



1st. The first is the Sclerotic* coat, thick, firm and 

 possessing but little sensibility. Its hardness gives secu- 

 rity to the delicate membranes beyond ; affords attach- 

 ment for the muscles ; and by its elasticity, equally distends 

 the ball, that none of the humors may suffer from pres- 

 sure. Happily the hard coat is very rarely diseased. 

 Fishes have a sclerotic coat strictly hard, being either 

 cartilaginous or firm bone, graduated in this respect ac- 

 cording to the depth to which they descend in search 

 of food. Without this compensation, the great weight of 

 the water above would crush in their eyes instantaneously. 

 Through this coat, in what is called the white of the eye, 

 the occulist plunges a needle to cure some kinds of blind- 

 ness. 



2d. Ckoroidi- is the name of the second coat, having 

 a dark red color, and apparently slightly connected with 

 the first. By carefully cutting off the sclerotic from a 

 bullock's eye, with scissors, the choroid will be beautifully 

 exhibited, sustaining the humors. Minute dissection, 

 under a microscope, shows this tunic is a complete web 

 of arteries and veins ; hence its reddish hue. Between 

 this and the sclerotic, fine silvery threads are seen, which 

 hold a control over the iris, yet to be described, deter- 

 mining by their influence how much or how little light 

 may safely be admitted into the eye. Fungous tumors 

 have their origin in this coat, growing so rapidly as to 

 burst the sclerotica, pushing their way out of their orbit 

 down upon the cheek, incorporating the whole ball in one 

 prodigious mass of disease. The inside of this membrane 

 resembles closely woven wailed cloth, having a fleecy 

 nap, similar to velvet, called tapetum.^. This tapetum is 

 particularly interesting in a philosophical point of view, 

 as on its shade of color, in a great measure, as will be 

 more fully explained in the sequel, depends the power of 

 seeing in the dark. 



3d. Retina,^ so called from its resemblance to a net, 

 completes the number, being the innermost and last. Its 



* Sclerotic, from a Greek word meaning hard. 

 t Choroides, like a lamb-skin, fleecy. 

 t Tapetum resembling cloth, called tapestry. 

 Retina a net. 



