310 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



The contents of the abdomen are not only adapted to the external form ; but 

 their direction in the cavity is, in some instances, regulated by it. The anterior 

 extremity, or fin, though formed of distinct parts, in some degree similar to the 

 anterior extremities of some quadrupeds, being composed of similar bones placed 

 nearly in the same manner, yet are so formed and arranged as to fit them for pro- 

 gressive motion in the water only. The external form of this order of animals is 

 such as fits them for dividing the water in progressive motion, and gives them 

 power to produce that motion in the same manner as those fish which move with 

 a considerable velocity. On account of their inhabiting the water, their external 

 form is more uniform than in animals of the same class which live on land ; the 

 surface of the earth on which the progressive motion of the quadruped is to be 

 performed being various and irregular, while the water is always the same. 



The form of the head, or anterior part of this order of animals, is commonly a 

 cone, or an inclined plane, except in the spermaceti whale, in which it terminates 

 in a blunt surface. This form of head increases the surface of contact to the same 

 volume of water which it removes, lessens the pressure, and is better calculated 

 to bear resistance of the water through which the animal is to pass ; probably, on 

 this account, the head is larger than in quadrupeds, having more the proportion 

 observed in fish, and swelling out laterally at the articulation of the lower jaw : 

 this may probably be for the better catching their prey, as they have no motion 

 of the head on the body ; and this distance between the articulations of the jaw is 

 somewhat similar to the swallow, goat-sucker, bat, &c. which may also be ac- 

 counted for, from their catching their food in the same manner as fish ; and this 

 is rendered still more probable, since the form of the mouth varies according as 

 they have or have not teeth. There is however in the whale tribe more variety 

 in the form of the head than of any other part, as in the whalebone, bottle-nose, 

 and spermaceti whales ; though in this last it appears to owe its shape, in some 

 sort, to the vast quantity of spermaceti lodged there, and not to be formed 

 merely for the catching of its prey. From the mode of their progressive motion, 

 they have not the connection between the head and body, that is called the neck,> 

 as that would have produced an inequality inconvenient to progressive motion. 



The body behind the fins or shoulders diminishes gradually to the spreading of 

 the tail ; but the part beyond the opening of the anus is to be considered as tail, 

 though to appearance it is a continuation of the body. The body itself is flat- 

 tened laterally ; and I believe the back is much sharper than the belly. The pro- 

 jecting part, or tail, contains the power that produces progressive motion, and 

 moves the broad termination, the motion of which is similar to that of an, oar in 

 sculling a boat ; it supersedes the necessity of posterior extremities, and allows of 

 the proper shape for swimming ; that the form may be preserved as much as 

 possible, we find that all the projecting parts, found in land animals of the same 



