338 PHILOSOPHICAL TRANSACTIONS. [anNO 1/87. 



passing out through it at the lower part, as they do at the upper, so that the 

 Cauda equina, as it forms, is on the outside of the dura mater. 



As the organs of sense are variously formed in different animals, fitted for the 

 various modes of impression; and as the modes are either increased or varied, 

 according to circumstances which make no part of the sense itself, but which are 

 necessary for the economy of the animal, we find the senses in this tribe varied 

 in their construction, and in some a sense is even wholly wanting. The organs 

 of sense, which appear to be adapted to every mode of life, are those of touch 

 and taste; but those of smell, sight, and hearing, probably require to be varied 

 according to circumstances. Thus smell may be increased by a mode of impreg- 

 nation, hearing by the vibration of different mediums, and sight by the different 

 powers of refraction of different mediums; therefore, as animals are intended by 

 nature to be differently circumstanced, so are the senses formed. 



Of the sense of touch. — The cutis in this tribe appears, in general, particularly 

 well calculated for sensation ; the whole surface being covered with villi, which 

 are so many vessels, and we must suppose nerves. Whether this structure is 

 only necessary for acute sensation, or whether it is necessary for common sen- 

 satiofi, where the cuticle is thick, and consisting of many layers, I do not know. 

 We may observe, that where it is necessary the sense of touch should be ac- 

 curate, the villi are usually thick and long, which probably is necessary, because 

 in most parts of the body, where the more acute sensations of touch are re- 

 quired, such parts are covered by a thick cuticle. Of this the ends of our 

 fingers, toes, and the foot of the hoofed animals, are remarkable examples. 

 Whether this sense is more acutein water, I am not certain, but should imagine 

 it is. 



Of the sense of taste. — ^The tongue, which is the organ of taste, is also en- 

 dowed with the sense of touch. It is also to be considered, in the greatest 

 number of animals, as an instrument for mechanical purposes; but probably less 

 so in this tribe than any other. However, even in these it must have been 

 formed with this view, since, merely 'as an organ of taste, it would only have 

 required surface, yet is a projecting body endowed with motion. In some it is 

 better adapted for motion than in others ; and I should suppose this to be re- 

 quisite, on account of the difference in the mode of catching the food, and in 

 the act of swallowing. It is most projecting in those with teeth, probably for 

 the better conducting the food, step by step, to the oesophagus ; whereas it does 

 not seem so necessary to have such management of the tongue in those which 

 have no teeth, and catch their food by merely opening the mouth, and swim- 

 ming upon it, or by having their prey carried in by the water. In the porpoise 

 and grampus it is firm in texture, composed of muscle and fat, being pointed 

 and serrated on its edges, like that of the hog. 



