AQUATIC MAMMALS 



high ridging, but on the contrary the occipital border is unridged and 

 superficial examination would give the impression of definite muscular 

 weakness in this area. The muscles need not be thick, or "powerful" in 

 the usual sense, but they are either shorter or the muscular fibers less 

 contractible, and the muscle must be tougher. This latter is best accom- 

 plished by the presence of numerous tendinous fasciculi or bundles, and 

 in the perfected state of this specialization, which no mammal has yet 

 attained, these occipital muscles would be nothing but tendinous sheets, 

 practically lacking muscle fibers, and resulting in the incapacity for 

 muscular movement of the head in a dorsal direction. 



Certain modifications of either A or B, or a combination of both, may 

 be effected mechanically by body conformation (a longer or shorter 

 neck, or size of head) , or by the provision, for instance, that has been 

 developed by some ungulates in quite perfected form. This may be well 

 demonstrated in a small antelope, which has need while grazing to lift the 

 head perhaps every half minute to watch for enemies. The great muscu- 

 lar energy that would otherwise be expended in this action during the 

 course of a day is conserved by the presence of a highly elastic nuchal 

 ligament extending middorsally to the head. If one props up a dead 

 individual in standing attitude, presses the head to the ground and then 

 releases it, the spring of the nuchal ligament will either automatically 

 raise the head or almost do so. This provision thus economically takes 

 the place of much more powerful cephalic muscles that would otherwise 

 be wasteful : but no aquatic mammal has this equipment. 



(C) Length of leg in a land mammal largely influences length of 

 neck (partially excepting browsers) . This, correlated with posture in- 

 duced by life habits, to some extent, determines the normal position for 

 carriage of the head. The giraffe, with its long neck, carries its head 

 with axis at a right angle to the neck, while many bovines carry the 

 cephalic axis practically in a line with those of the neck and body. This 

 naturally has an effect upon the occipital musculature. The pig, while 

 rooting, begins to apply the muscular force necessary when the cephalic 

 axis is almost at a right angle to the neck. If the occipital plane were 

 tilted forward or even vertical to the skull axis the occipital muscles 

 would have no effective lever arm on which to work. Therefore it is 

 mechanically necessary for the slope of the occcipital plane to be toward 

 the rear, which it is to a very marked extent. The projection of the lamb- 

 doidal crest but accentuates this feature. Surely the head and neck of a 

 horse is not as strong as of a large bull, but the horse has an occipital 

 plane caudally tilted and prolonged into a large crest, and so exhibits 



[130] 



