FUNCTIONS OF QUADRUPEDS. 17 



all the parts, this must here be omitted. The elongation of 

 the brain, through the large hole in the base of the skull, is 

 denominated medulla spinalis, or spinal marrow. This, 

 externally, appears to be composed of a white substance; 

 but internally it assumes a greyish tinge. Its form is that 

 of a cylinder somewhat compressed ; and it seems to be com- 

 posed of two cords, divided on each side by a furrow. The 

 spinal marrow gives origin to as many pairs of nerves as 

 there are holes between the vertebrae. In the human sub* 

 ject, nine pairs of nerves arise from the brain, and thirty* 

 one from the spinal marrow. 



The first pair of nerves issuing from the brain are those 

 which constitute the organ of smell. The second, called the 

 optic nerves, are the principal organ of sight. The third, 

 the fourth, and the sixth pairs, serve to move the muscles of 

 the eye. The fifth, which are large, and each divided into 

 three branches, are distributed over the eyes, eyelids, fore- 

 head, nose, and the integuments of the face. The seventh 

 pair constitute the organ of hearing. The eighth extend 

 over the interior parts of the thorax or chest, and abdomen; 

 and the ninth pair extend to the tongue, where they con- 

 stitute the organ of taste, and likewise contribute to the 

 motions of that member. The thirty-one pairs of nerves 

 which issue from the spinal marrow are chiefly distributed 

 to the exterior parts of the trunk, and to the extremities. 



The eye is a spherical body, composed of various coats, 

 filled with different kinds of humours, and moved, within 

 the socket that contains it, by means of muscles. The ex- 

 ternal coat is a .thick, opaque, and whitish membrane, 

 called the schlerotis; but its anterior part is transparent, - 

 and forms the segment of a smaller sphere, called the cornea. 

 Under this lies the choroid coat, a fine membrane, covered 

 with numerous blood vessels, and coloured interiorly, in the 



fjiuman subject, by a blackish pigment. Jt terminates in 

 C front 



