184 PROF. JOHN CLELAND, M.D,, ETC.^ ON 



vertebrates, they each spring from a girdle, present a single 

 boned upper part, constituting the arm or thigh, and beyond 

 this a forearm or leg as the case may be, which is followed 

 by a hand or foot of omiplex structure, exhibiting usually 

 more or less distinctly the multipartite carpus or tarsus 

 surmounting digits uot more than five. That general 

 description is applicable to Amphibia but not to Fishes ; 

 limbs easily compared in detail with our own exist in the 

 former, but not in the latter. Also there is no ascertained 

 correspondence of the nerve-supply of the limbs of fishes 

 with those of other vertebrates, and it may indeed be stated 

 that the ventral fins or hind limbs are obviously supplied 

 by different nerves in difterent fishes, while any one can 

 appreciate the similarity of the nerve-supply of the limbs of 

 amphibians, reptiles, birds and mammals. 



Thirdly, the auditory organ of fishes consists simply of an 

 internal ear, and when accessory apparatus is developed to 

 regulate the pressure on the internal ear it is in the form of 

 a modification of the swimming bladder and of ribs and 

 transverse processes of vertebrae. Not so when we leave the 

 fishes. Accessory apparatus becomes the rule instead of the 

 exception, and it is still more important to observe that the 

 regulation of pressure is always eft'ected by an element 

 termed columella in amphibians, reptiles and. birds, — the 

 stapes of mammals. 



Fourthly, it is already in the Amphibia that we see for the 

 first time the head separated from the trunk by a neck, or 

 region intervening between the head in front and the heart 

 and lungs behind. 



In all these particulars of large structural or functional 

 importance the amphibia cohere to the reptiles and are 

 separated from the fishes. 



Turn to the Birds. They are so separated from Mammals 

 that biologists have ceased to look for a direct link uniting 

 them with these, and the only question is as to the importance 

 of the gap dividing them from Reptiles. Now it is certainly 

 a thing not to be overlooked or made little of, that the 

 fieparation of the pulmonary from the systemic circulation, 

 toward which the reptiles show so many stages of anatomical 

 progress, is complete in all birds and mammals ; and not 

 unconnected with this is the circumstance that these alone 

 are the warm-blooded animals. Further they difter from 

 reptiles in having their integuments protected by horny 

 epidermal growths each lounded on a single papilla, for this 



