4:76 Lull — Functions of the '^Sacral Bra'm^^ in Dinosaurs. 



The spinal canal of Stegosaiirus ungulatus has been 

 studied in detail by the author, who finds not only a 

 sacral dilatation but a brachial one as well ; that is, from 

 vertebrsB VIII to XIII, the maximum width, that of 38 

 mm., as compared with the average of 25 mm., is attained 

 by vertebra XI, which is exactly opposite the shoulder 

 articulations in the Yale mounted specimen. There is 

 also a corresponding heightening of the canal, although 

 this is a less constant feature, for further back (XV and 

 XVII) there is evidence of a ligament or other delimiting 

 structure below the bony roof of the neural arch itself. 

 Brachial and sacral dilatations of the neural canal are 

 most marked in the turtles among existing reptiles, 

 owing to the immobility of the trunk and the consequent 

 reduction of its musculature and associated nerves, the 

 two enlargements being necessary where the nerves 

 depart to the limbs. That this is the whole significance 

 of these two enlargements in Stegosaiirus and also in 

 other dinosaurs I have no doubt, and the relative size of 

 each dilatation bears an approximate ratio to that of the 

 limbs innervated, plus in the hinder pair the huge caudo- 

 femoral and other muscles which actuated the tail. 



I still feel, despite the contention of the German 

 writers, that the ^* sacral brain" — ^^vhich should not be 

 called by such a term — ^possessed no unusual function 

 whatever, but only the normal one of transmission and 

 reflex action in an unusual degree, and that to invoke any 

 new and unknoT\m function as a reason for its relatively 

 immense size, especially one connected with digestive 

 efficiency, is not justified by the evidence at hand. 



Branca further says:^ '^We may also think of these 

 animals as sluggish in habit, in consequence of which 

 much less food was required than is the case in an active 

 animal." On the other hand, in warm-blooded animals 

 the largest species occur in cooler climates, because large 

 animals have ^^a relatively smaller radiating surface 

 than smaller ones, a factor of the greatest importance in 

 the regulation of body warmth." To the first statement 

 I can take no exception. The second, however, gives 

 food for thought. In the first place, is it an invariable 

 rule that the largest species of warm-blooded animals 

 occur in cooler climates? The present-day distribution 



*Loc. cit. 



