126 Frogs, and their Contributions to Science. 



course of the femoral nerve, made up as it is on each 

 side by four filaments given oft' from the spinal cord at the 

 last four vertebras, and passing down on the outside of the 

 ilia, permit its easy isolation, and the making of it for 

 purposes of experimental inquiry, the sole channel of com- 

 munication between the lower extremities and the trunk. 

 The nerves of special sense, those of sight, hearing, taste, 

 and smell, are easily traced. Sensation is more perfect 

 about the head than the body. The eye is comparatively 

 large, promiuent and lustrous, and is beautiful to look at, 

 or study. Its iris is not dilatable, so far as I can perceive. 

 Besides two palpabrse, it is guarded by a semi-transparent 

 membrana nictitans. The ear has no external orifice. A 

 scale like membrane covers the auditory canal, like a drum- 

 head, and communicates its vibrations to the nerve. The 

 sense of sight is keen, that of hearing good, while those 

 of taste and smell, from anatomical reasons, are not deemed 

 sources of very exquisite enjoyment. 



In the early tadpole portion of his life, the frog respires 

 like a fish, by the aid of branchiae or gills, but in the pro- 

 gress of his development, these, with the single heart 

 connected with them, give place to the true lungs and 

 the true heart, having a ventricle and two auricles. From 

 the manner in which the circulation is thereby effected, 

 the venous and arterial currents being mingled, aeration 

 of the blood is but imperfectly accomplished. The frog 

 breathes through his nose, never by his mouth. The air 

 is drawn in by the expansion of the muscles of the throat 

 and forced by their contraction, the nostrils being plugged 

 as it were by the tongue on the inside, into the lungs. 

 Expiration is effected by the muscles of the abdomen. 

 The want of ribs in the formation of the thoracic cavity 

 make this process of swallowing the air a necessity. The 

 number of respirations is about eighty per minute, the 

 aeration of the blood being as already stated greatly aided 



