354 THE SENSORIAL FUNCTIONS. 



channel above described, which has the effect of a pulley, is 

 conducted through a circular sheath, furnished by the scle- 

 rotica to the under part of the eye, and is inserted into the 

 lower portion of the loose edge of the nictitating membrane. 

 By the united action of these two muscles, the former of 

 which serves merely to guide the tendon of the latter, and 

 increase the velocity of its action, the membrane is rapidly 

 drawn over the front of the globe. Its return to its former 

 position is effected simply by its own elasticity, which is 

 sufficient to bring it back to the inner corner of the eye. 

 If the membrane itself had been furnished with muscular 

 fibres for effecting this motion, they would have interfered 

 with its use by obstructing the transmission of light. 



The eyes of quadrupeds agree in their general structure 

 with those of man. In almost all the inferior tribes they are 

 placed laterally in the head, each having independent fields of 

 vision, and the two together commanding an extensive por- 

 tion of the whole sphere. This is the case very generally 

 among fishes, reptiles, and birds. Some exceptions, indeed, 

 occur in particular tribes of the first of these classes, as in 

 the Uranoscopus, where the eyes are directed immediately 

 upwards; in the Ray and the Callionymus, where their di- 

 rection is oblique; and in the Pleuronectes, where there is 

 a remarkable want of symmetry between the right and left 

 sides of the body, and where both eyes, as well as the mouth, 

 are apparently situated on one side. Among birds, it is only 

 in the tribe of Owls, which are nocturnal and predacious, 

 that we find both eyes placed in front of the head. In the 

 lower quadrupeds, the eyes are situated laterally, so that 

 the optic axes form a very obtuse angle with each other. 

 As we ascend towards the quadrumana we find this angle be- 

 coming smaller, till at length the approximation of the fields 

 of view of the two eyes is such as to admit of their being 

 both directed to the same object at the same time. In the 

 human species the axes of the two orbits approach nearer to 

 parallelism than in any of the other mammalia; and the fields 

 of vision of both eyes coincide nearly in their whole extent. 



