THE SKULL 



much greater "strength", from the usual but erroneous standpoint, than 

 does the bull. The lambdoidal crest of the horse is largely due to the 

 mobility of the head, and the backward inclination of the occiput to the 

 fact that the head is usually held at a sharp angle with the neck. Even 

 more strikingly illustrative of these points is the condition in man and 

 other primates which hold the body erect for most of the time. Hence 

 it is a law that in those mammals which habitually hold the axis of the 

 head at near a right angle to that of the neck, the occipital plane is found 

 to slope backward, to a greater or lesser degree according to the strength 

 of possible antagonistic stimuli. 



(D) Among the Bovidae conditions opposite to those mentioned 

 above are perhaps best exemplified in the water buffalo. In observing 

 a living specimen one is immediately struck by the fact that it carries 

 the axis of the head in almost a straight line with that of the neck, and 

 that it does so almost continuously. We know that it has far more 

 strength in this region than the horse, but there is no lambdoidal crest 

 and the occiput shows some tendency to slope forward rather than to 

 the rear. In the more strongly modified fossorial rodents this occipital 

 tilting is strikingly marked. Those which can still see never have occa- 

 sion to look in a ventral direction, but only above, to watch for an enemy 

 while the head is level with the burrow entrance. The head is held 

 on a line horizontal with the body, in rest, when tamping the burrow 

 walls with the nose (as Spalax), and even above the body when the 

 teeth are being used as a pick while extending the burrow system. Ac- 

 cordingly the tilting of the occipital plane is most pronounced in those 

 forms which have been subjected to fossorial conditions for the longest 

 time, as indicated by their other adaptations in this direction. These 

 rodents have great bulging occipital muscles but usually a very slight 

 lambdoidal crest. The reason for the lack of the latter is probably partly 

 because movements, although they may be of great strength, are limited 

 in distance, that a good lever arm is secured without the necessity of a 

 high crest, and, as seems probable, that a crest is not nearly so liable 

 to develop upon the apex of an obtuse as of an acute bony angle. From 

 contemplation of the above facts it would seem that when the head is 

 normally carried parallel with the axis of the neck the occipital plane 

 will be tilted in an anterior direction. 



(E) Certain developments that I have noted only in Cetacea would 

 seem to point to the possibility of a fifth set of conditions that could 

 be of import in influencing occipital conformation. In Balaenoptera 

 boreal is Schulte (1916) showed a single semispinalis capitis muscle as 



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