406 THE MECHANICAL FUNCTIONS. 



frequently found at the exterior part of the angles 

 which they form by their junction, serving the 

 purpose of giving a more advantageous position 

 to the tendons of the muscles which extend 

 those joints. The patella, or knee pan, (K), is 

 the largest of these, and is pretty constantly 

 present. Smaller bones of this description are 

 met with on the joints of the ringers, and are 

 termed sesamoid bones. 



On comparing these divisions of the limbs of 

 quadrupeds with those of insects we cannot fail 

 to perceive that there exists between them a 

 marked analogy : and that naturalists were not 

 led away by mere fancy when they applied to 

 the latter the same names as those borne by the 

 former. This, however, is not the only instance 

 of analogy that may be discovered between the 

 structures of articulated and of vertebrated ani- 

 mals, however strong may be the contrast which 

 they offer in all the essential features of their 

 conformation. The rings which compose the 

 skeleton of the insect, and which enclose its prin- 

 cipal nervous chords, have been supposed to have 

 an analogy with the circles of bone which consti- 

 tute the primary forms of the vertebra?, and which 

 contain the spinal chord ; although in the first 

 case, it is true, other viscera are included within 

 the arches, whereas none are contained in the 

 last case. They agree, also, in having the head 

 placed at one extremity, distinct from the trunk, 



