100 “TERRA NOVA” EXPEDITION. 
This alteration was brought about by the raising and lowering of the convexity of 
each ramus, simultaneously, and respectively with the shutting and opening of the 
mouth. The movement of the rami necessarily involves the elasticity of the 
symphysis, which joins the two rami together at their anterior extremities. The 
structure of the symphysis, upon examination, was found to be almost identical with 
that of the intervertebral cartilages. ' 
It has been suggested that a whale swims by means of movements of the tail- 
flukes, somewhat after the manner of a steamer which is forced through the water by 
its propeller. If this were true it would imply that the flukes of the tail had 
independent muscular movements of their own. Now this is surely impossible, since 
the flukes are entirely composed of white fibrous tissue and blubber, and contain no 
trace of muscular tissue or tendons. 
Moreover, I have often watched the tail-flukes of the Piked Whale and of various 
dolphins in the act of swimming, and have seen no other movement in the flukes than 
that which takes place in the caudal fin of fishes. In the Cetacea it would seem that 
the tail as a whole moves up and down when the animal is progressing, and the motion 
appears to be very similar to that which takes place in the tail of a fish, except that 
in the latter the movement is from side to side. 
For this reason, the backbone of a whale has to be especially flexible at the 
posterior end, in order to allow the powerful tail, which is the principal organ of 
propulsion, to have free play, Consequently the vertebrae have imitated, as it were, 
those of fishes, in substituting large intervertebral discs for the imterlocking, bony 
processes of ordinary land vertebrates. These discs consist of a broad ring of white 
fibrous tissue surrounding a central core of jelly-like pulpy substance, the nucleus 
pulposus. 
Precisely the same structure occurs in the symphysis of the lower jaw (text- 
fig. 5, B). The thick ring of tough, elastic tissue surrounding a ball of jelly forms an 
admirable hinge, and permits the necessary movement of the rami when the whale 
opens and shuts its mouth. The amount of hinge-movement required in this case is 
about equal to that of the intervertebral discs of the lumbar region. 
A similar symphysis occurs in the mandible of Balaenoptera; and the above- 
mentioned movement of the rami is probably characteristic of Whalebone Whales in 
general. 
The reason for this movement of the lower jaw is undoubtedly to allow of an 
increase in the straining surface of the baleen. As shown in text-fig. 5, A, a, the 
plates of baleen curve outward at their distal ends, and project beyond the margin of 
the upper jaw to the extent of about a foot. By this means some eight square 
feet are added to the straining area in a full-sized Humpback. 
The outward movement of the convexity of the rami enables the mandible to 
clear the projecting baleen-plates, when the mouth is opened and closed. 
