THE NECK 



occipital muscles are evenly distributed along the lambdoidal area to 

 facilitate diversity of head movements. 



In the case of an aquatic mammal as highly specialized as the seal or 

 whale, in which the propelling mechanism is situated at the hinder end, 

 an entirely different set of physical laws is introduced. For proper effi- 

 ciency it is obliged to be of fusiform shape, just as must an airship of 

 the Zeppelin type, the fuselage of an airplane, or (with proper camber) 

 the cross section of its wing. We are entirely justified in accepting it 

 as an incontrovertible fact that with this type of body and of propulsion 

 a long, mobile neck would be impracticable, and that the neck form or 

 its musculature or both must be such that when swimming at speed, 

 this part of the animal is included within the uninterrupted contour of the 

 fusiform body. There must be a point constituting the center of equi- 

 librium, which may also be the pivot of motion, situated anterior to the 

 middle of the body and theoretically this should be at that portion of 

 the thorax having the greatest circumference, which will usually fall 

 at the shoulders. In the Phocidae, especially, the pivotal point of swim- 

 ming motion is fixed in this region by the fact that here are anchored 

 the lateral muscles comprising the power arm of the tail in one direction, 

 and of the neck and head in the other. Now for most effective results 

 for the muscular power expended it is absolutely essential that there be 

 sufficient mass to the prethoracic part, while if the mass be too great, 

 then efficiency is reduced. 



Breder (1926) considered that in most fish the pivotal point lies 

 through the atlas, and this may very well be correct. As discussed in 

 a previous chapter it is deemed that there may be a single pivot of motion 

 coinciding with the center of equilibrium, or in an animal in whose swim- 

 ming less of the tail is involved there may be two pivots of motion, one 

 posterior and one anterior to the center of equilibrium. In the seal, 

 however, with its relatively small head, it seems that the pivot of motion 

 (or pair of pivots) is situated in the region from which the lateral 

 lumbo-caudal and cervico-cephalic impulses arise. Some of these muscles 

 may shift their points of anchorage to the thorax over the distance of 

 one, two, or three intercostal spaces, or those to the arm may wander 

 slightly, but on the whole their possible migration is very limited. 



In general principles the body of the seal (Phocidae) is subjected 

 while swimming to the same laws encountered by whales. We might 

 therefore expect to find in the former the same marked tendency toward 

 a shortening of the cervical series as characterizes the latter group. On 

 the contrary, however, it may seem somewhat surprising to find that in 



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