He finds that each of these simple eyes is composed, 1st, of a cornea; 2dly, of a crys talline lens ; 3dly, of a vitreous body; 4thly, of a kind of chamber; 5thly, of a choroid; 6thly, of a retina.
The cornea, as is shewn in fig. 94, which represents a vertical section of the eye, is smooth and con vex externally (a,) its superficies pre , senting none of those divisions which characte r rize the cornea of the compound e eyes of insects.
The internal sur face is deeply concave, and in the hollow he misphere thus formed is lodged the anterior part of the crys talline lens. This body (b) is of a spherical figure, of a hard and transparent texture, resem bling in these respects the crystalline lens of Fishes. Posteriorly it rests upon but does not penetrate the vitreous humour. The vitreous humour (c) is composed of a granular, soft material, is larger than the chrystalline, plano convex anteriorly, wholly convex behind. As the crystalline lens rests upon without sinking into the vitreous humour, there remains a cir cular channel or space filled with an aqueous humour, to which the term chamber may be appropriately given, and which may be com pared to the posterior chamber of the eye of some of the vertebrata.
The retina (e) is applied to the back part of the vitreous humour, and is in some degree an expansion of the optic nerve (g). It is lined by a choroid, or membrane saturated with a co loured matter, or kind of pigmentum (f), which is afterwards reflected over the anterior margin of the plano-convex surface of the vitreous humour so as to form there a sort of pupil, the aperture of which exceeds the diameter of the crystalline, but is less than that of the vitreous humour. Such is the somewhat complicated structure of one of the large eyes of a scorpion, by the knowledge of which physiologists are now enabled better to understand the mode in which vision is effected in the arachnidans.
Organs of secretion.—We designate thus the organs that emit outwardly a matter which is sometimes liquid, and sometimes becomes con crete by contact with the atmosphere. The position of these organs varies ; in one case they occupy the anterior part of the body, in another they are observed at the opposite extre mity. The nature and properties of the matter secreted is not less variable; in some instances it is an irritating or poisonous liquid which the animal introduces by means of a more or less sharp pointed hook into the interior of the body to which it may be applied; in other instances, again, it is a substance which is at first in a liquid state, but soon becomes solid in its passage through a sort of sieve, or, if I may be permitted the comparison, a cullender pierced with excessively minute holes. We shall treat
separately of these two kinds of apparatus.
Of the apparatus for secreting the irritating or poisonous liquid.—Every one knows how quickly a fly that has been bitten by a spider expires : the effect is instantaneous. It is by means of the mandibulm or forciples that the spider has inflicted the wound. These mandi bulee are each armed with a moveable and ex tremely sharp claw, (fig. 95, a,) near to the point of which is a minute orifice (b), from which there escapes a drop of poisonous liquid, which spreads itself over the whole wound the instant that it is inflicted. This orifice, which from its minuteness is very difficult to be perceived even with a high magnifying pow er, communicates with a fine or narrow excretory canal (c), situated in the interior of the d mandible and given off from the true secreting organ. This gland is lodged in the inter space of the muscles of the thorax; it is in the form of an elongated and slightly curved vesicle, the parietes of which have a singular structure. Trevimnus describes it as consisting of filaments adhering together and united by a membrane so as to resemble a spirally disposed band. This structure presents, he thinks, some analogy to that of the trachea of insects. Ly onnet, in his posthumous work, has described this part somewhat differently : he considers each little band as being composed of two sub stances, one fleshy, which contracts upon drying, the other squamous, which is disposed like a watch-spring, or rather like Archimedes' screw, and which always remains in the same state. He supposes that these fibres, upon contracting, force the poisonous liquid into the excretory canal. Such a construction is not, however, necessary, since it may be readily conceived that that vesicle, being placed in the midst of very powerful muscles, it is sufficient that they contract in order to its compression and the consequent propulsion of the fluid contained in its interior, which probably the parietes have secreted.