THE SKULL 



In all cetaceans there has been a mechanical stimulus for the quantita- 

 tive shortening of the neck, and this has been accompanied by a shorten- 

 ing of the muscles extending from the thorax to the head. This acts 

 in anchoring the latter more firmly, which is of decided advantage to 

 the animal. In a massive body which moves through the water with 

 the velocity characteristic of whales of less extreme specialization, any 

 really marked lateral movements of the relatively large head might be 

 as disastrous as is the abrupt turning of the front wheels of an automobile 

 running at speed. It would enable the pressure of the water to snap the 

 head sharply to the side and might result in a broken neck. A whale 

 does not need any movement of the head in a sagittal plane for steering, 

 because this is accomplished by tilting the flippers. It swims by inducing 

 vertical curvature of the entire body, usually from a point in the anterior 

 thorax as already explained, and this necessitates slight movements of 

 the head in this plane. This is a constantly repeated action, whereas 

 slight lateral movements of the head for steering in the horizontal plane 

 (also facilitated by flipper action) are only occasional. As already stated, 

 these lateral movements must be moderate in degree and should be, as 

 they are, but little if any more decided than the possible curvature of 

 the body proper. In a right turn, for instance, the muscles of the right 

 side need apply practically no power. What is principally needed is a 

 relaxation of the muscles of the left side of the neck, when the water 

 pressure against the left side of the rostrum will force the head toward 

 the right. Then must the left muscles have great static strength to keep 

 the head from turning too far, and active strength through a short 

 distance only to overcome the water pressure and bring the head back 

 into position when it is wished to progress once more in a straight line. 



In addition to essential but short sagittal movements of the head in 

 swimming most odontocetes probably have need for tilting the head 

 downward in securing food. The amount of this is unknown, but at 

 least we know that in almost all sorts the rostrum is carried somewhat 

 more depressed and is not directly on a line with the body axis. We 

 would therefore not expect to find the occipital plane so sharply tilted 

 forward as would be the case were the rostral axis parallel with the 

 body. Furthermore, in most odontocetes the rostrum is relatively not 

 nearly so large as in mysticetes and the occipital musculature does not 

 need such leverage to control its movements. In considering this detail, 

 however, there is encountered a great complexity of possible stimuli 

 which it is difficult to explain with clarity, and almost impossible at the 

 present time properly to evaluate. 



[133] 



