_Alice._ I understand now why disease of the spine causes such involuntary contortions and gestures, in some instances. Its connection with the brain and nerves is so immediate, that it cannot suffer disease without affecting the whole nervous system. _I._ It cannot. The spinal cord or marrow is a continuation of the brain. But we must not devote any more time to this subject. _Bertha._ I want to ask you something about the different parts of the eye, Isabel. When ---- ---- lectured on optics, I lost nearly all the benefit of his lecture, except a newly awakened desire for knowledge on this subject. He talked of the retina, cornea, iris, &c.; please tell me precisely what they are. _I._ The retina is a nervous membrane; in other words a thin net-work, formed of very minute sensitive filaments. It is supposed by some to be an expansion of the optic nerve; and on this the images of objects we see are formed. It is situated at the back part of the eye. Rays pass through the round opening in the iris, which we call the pupil. _B._ What did the lecturer say is the cause of the color of the pupil? _I._ He said that its _want of color_ is to be imputed to the fact that rays of light which enter there are not returned; they fall on the retina, forming there images of objects. And you recollect he said that "absence of rays is blackness." The iris is a kind of curtain, covering the aqueous humor--aqueous is from the Latin _aqua_, water. It is confined only at its outer edge, or circumference; and is supplied with muscular fibres which confer the power of adjustment to every degree of light. It contracts or dilates involuntarily, as the light is more or less intense, as you must have observed. The rays of light falling on that part of the iris which immediately surrounds the pupil, cause it to be either black, blue, or hazel. We will not linger on this ground, for it belongs more properly to Natural Philosophy. We will discuss the other four senses as briefly as possible. "The sense of taste," says Hayward, "resides in the mucus membrane of the tongue, the lips, the cheeks, and the fauces." Branches of nerves extend to every part of the mouth where the sense of taste resides. The fluid with which the mouth is constantly moistened is called mucus, and chiefly subserves to the sense of taste. _Ann._ I have observed that when the mucus is dried by fever, food is nearly tasteless. I now understand the reason. _E._ _Apropos_ to the senses, let me ask if feeling and touch are the same. Alfred says they are; I contend they are not, precisely.