1002 ON FORM AND MECHANICAL EFFICIENCY [ch. 



by the vertebral column and its ligaments alone, but that the pelvis, 

 firmly united as it is to the sacral vertebrae, and stretching back- 

 wards far beyond the acetabulum, becomes an intrinsic part of the 

 system ; and heljnng (as it does) to carry the load of the abdominal 

 viscera, it constitutes a great portion of the posterior cantilever arm, 

 or even its chief portion in cases where the size and weight of the 

 tail are insignificant, as is the case in the majority of terrestrial 

 mammals. 



Tail Head 



Fig. 479. Stress- diagram of backbone of Dinosaur. 



We may also note here, that just as a bridge is often a " combined" 

 or composite structure, exhibiting a combination of principles in 

 its construction, so in the quadruped we have, as it were, another 

 girder supported by the same piers to carry the viscera; and con- 

 sisting of an inverted parabolic girder, whose compression-member 

 is again consti'tuted by the backbone, its tension-member by the 

 line of the sternum and the abdominal muscles, while the ribs and 

 intercostal muscles play the part of the web or filling. 



A very few instances must suffice to illustrate the chief variations 

 in the load, and therefore in the bending-moment diagram, and 

 therefore also in the plan of construction, of various quadrupeds. 

 But let us begin by setting forth, in a few cases, the actual weights 

 which are borne by the fore-limbs and the hind-limbs, in our 

 quadrupedal bridge*. 



* I owe the first four of these determinations to the kindness of Sir P. Chalmers 

 Mitchell, who had them made for me at the Zoological Society's Gardens; while 

 the great Clydesdale carthorse was weighed for me by a friend in Dundee. 



