336 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



ings form a passage for the air in respiration to and from the lungs; for it 

 would be impossible for these animals to breathe air through the mouth ; indeed 

 I believe the human species alone breathe by the mouthy and in them it is 

 mostly from habit; for in quadrupeds the epiglottis conducts the air into the 

 nose. In the whole of this tribe, the situation of the opening on the upper 

 surface of the head is well adapted for this purpose, being the first part that 

 comes to the surface of the water in the natural progressive motion of the 

 animal ; therefore it is to be considered principally as a respiratory organ, and 

 where it contains the organ of smell, that is only secondary. 



As the animals of this order do not live in the medium which they inspire, 

 the organs conducting the air to the lungs are in some sort particularly con- 

 structed, that the water in which they live may not interfere with the air they 

 breathe. The projecting glottis, which has been described, passes into the poste- 

 rior nostrils, by which means it crosses the fauces, dividing them into 2 passages. 

 The enlargement at the termination of the glottis, observed in some of them, 

 would seem to be intended to prevent its retraction; but, as it seems confined 

 to the porpoise and grampus, it may perhaps in them answer some other 

 purpose. 



The beginning of the posterior nostrils, which answers to the palatum molle 

 in the quadruped, having a sphincter, the glottis is grasped by it, which renders 

 its situation still more secure, and the passages though the head, across the 

 fauces and along the trachea, are rendered one continued canal; this union of 

 glottis and epiglottis with the posterior nostril, making only a kind of joint, 

 admits of motion, and of dilatation and contraction of the fauces in deglutition, 

 from the epiglottis moving more in or out of the posterior nostril. This con- 

 struction of parts answers a purpose similar to that of the epiglottis in the quad- 

 ruped; it may be considered as the epiglottis and the arytenoid cartilages joining, 

 to make a tubular or cylindrical epiglottis^ instead of a valvular one. The rea- 

 sons why there should be so peculiar a construction of parts do not at first 

 appear; but we certainly see by it an absolute guard placed on the lungs, that 

 no water should get into them. This tribe being without the projecting tongue 

 of the quadruped, and wanting its extensive motion, and the power of sucking 

 things into the mouth, may probably require the construction between the air 

 and lungs to be more perfect; but how far it is so, I will not pretend to say. 



The size of the brain differs much in different genera of this tribe, and like- 

 wise in the proportion it bears to the bulk of the animal. In the porpoise I 

 believe it is largest, and perhaps in that respect comes nearest to the human. 

 The size of the cerebellum, in proportion to that of the cerebrum, is smaller in 

 the human subject than in any animal with which I am acquainted. In many 

 quadrupeds, as the horse, cow,&c. the disproportion in size between cerebellum and 



