Ixxix 



and with less fatigue, than the quadruped or man, who are in- 

 capable of keeping up long-continued rapid progression, not 

 so much from debility in the muscular system generally, as 

 from a failure in the inspiratory muscles, which, under such 

 exciting and exhausting circumstances, are called on to exert 

 additional force, in order to maintain the due quality of the 

 blood, as well as to regulate the current of the circulation, 

 and which exertions, when too long continued, are speedily 

 followed by that overwhelming and well-known, though al- 

 most indescribable, sensation denominated fatigue. It appears 

 to me, also, that this organization may still further minister 

 to the respiratory function, by extending the surface of the 

 mucous membrane on which the chemico-vital changes in the 

 blood are effected. The lining membrane of this reservoir 

 presents not only a very extensive surface, but it is also as 

 highly organized as that lining the trachea and bronchial 

 tubes, with which it is continuous ; numerous capillaries and 

 tortuous nerves branch throughout its texture, and several 

 large veins course irregularly along its wall ; it is, thereforej 

 highly probable that the same changes which are effected in 

 the blood through the parietes of the minute pulmonary ca- 

 pillaries, and through the thin coats of the large veins which 

 traverse the air-cells in the bodies of birds generally, may all 

 take place on the lining membrane of this cervical air-bag ; 

 and that this additional respiratory agency will be supplied at 

 that very time when the function of respiration is required in 

 the highest degree to maintain the muscular exertion and the 

 nervous energy which the animal evinces during its rapid ex- 

 cursions. This adjunct to the respiratory apparatus may 

 even be the more necessary to this animal as a compensation 

 for the imperfectly-developed, or almost rudimental wings, 

 which are not only of little or no avail in locomotion, but 

 which, from the absence of those large air-cells and blood- 

 vessels which exist in the wings of other birds, can here in 

 no way contribute to the respiratory function. 



