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 
froma 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, therefore, 
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- 
eursions. 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. 
