IGUANAS. 95 



panded broadly during excitement. It does not, 

 however, form a bag, nor is it susceptible of infla- 

 tion, as commonly represented, but is simply 

 extended by means of the branches of the tongue- 

 bone {os hyo'ides), which in repose pass down lon- 

 gitudinally beneath the skin. Under the in- 

 fluence of excitement, the tips of this bony fork 

 are drawn downward and forward, stretching the 

 skin with them, in an elliptical form, the two 

 inner surfaces of the skin being in contact.* This 



* The apparatus by which this extension is effected was thus de- 

 scribed as long ago as 1825, by Professor Bell. " The frame- work of 

 this structure consists of a remarkable developement of the os hyoides 

 or bone of the tongue. This bone is situated immediately under the 

 larynx. It has two long, slender, bony processes on each side, which, 

 for the sake of distinction, I shall term the anterior and posterior 

 lateral processes, — a small one which is filiform, given off from the 

 anterior part of the bone, and closely connected with the under part 

 of the tongue, — and a very long, delicate, and elastic cartilage, extend- 

 ing from the body of the bone, nearly to the middle of the abdomen, 

 immediately beneath the skin, to the internal surface of which it is at- 

 tached by condensed cellular tissue. In its form it is slightly flat- 

 tened and tapering to its extremity, where it is extremely slight and 

 flexible. 



" The anterior lateral process extends backwards to the angle of the 

 lower jaw, over the muscles of which it is curved upwards. The 

 posterior lateral process is placed in a similar direction, but does not 

 take the same curve over the edge of the jaw. These two processes 

 rmi parallel to each other through almost theii* whole length, but at a 

 small distance apart. 



" There are several pairs of muscles attached to these little bones, by 

 the action of which the phasnomena are produced, which it is my ob- 

 ject to explain. Of these the following are the principal. A broad 

 strong muscle arises from the whole edge of the posterior lateral pro- 

 cess, and is inserted into the clavicle. It is consequently of consider- 

 able length ; and by its contraction the whole bony frame- work is 

 drawn back and a little downwards, so that the distance between the 

 two extremities of the long elastic cartilage being diminished, this 

 organ is thrown into a curve, and the skin of the throat and belly is 

 stretched upon it, exactly as the silk is strained upon the whale-bone 

 ribs of an opened umbrella. The parts are restored to their natural 

 state by the action of [counteracting] muscles." (Zool. Journ. ii. 12.) 



