THE RESPIRATORY APPARATUS IN BIRDS. 479 



membranous folds which, partition its cavity. The membrane forming it being continued 

 on itself, every organ traversing the thorax becomes the cause of a fold in which it is 

 imprisoned ; and as the thoracic cavity is traversed by the trachea and the oesophagus, the 

 muscles which move the inferior larynx, and the arteries and veins, it will be understood 

 how this reservoir should become irregular in consequence of these various partitions, 

 and nlso why the other serial sa"s situated between the viscera and the walls of the 

 thorax, or the simple contiguous surfaces, should preserve their regular and proper 

 form. 



" The thoracic reservoir communicates with the lungs by an infundibular orifice, 

 situated on the external side of the embouchure of each bronchus. This orifice is 

 dilated during inspiration, by the contraction of the two first fasciculi of the pulmonary 

 diaphragm." 



2. Cervical reservoirs (Fig. 246, 1, 1). " They are situated above the preceding, 

 and the inferior part of the neck and anterior part of the lung ; inflated after removal 

 from the neighbouring parts, they resemble two cones, whose rounded base looks 

 forwards, and whose pediculated summit is directed backwards. 



" Superiorly, these reservoirs lie against the cervical muscles ; inferiorly, they corres- 

 pond to the air-sac of the thorax, from which they are separated by the trachea, the 

 oesophagus, the pneumogastric nerves, and the jugular veins. Inwardly, they are in 

 juxtaposition, and consequently form a median septum which includes in its substance 

 the two common carotid arteries. Outwardly, they are related to the origin of the 

 cervical nerves, to each of which they furnish a small sheath, and with the vertebral 

 artery which they surround, but do not contain in their cavity, as well as with a 

 subcutaneous muscle and the skin. By their summits, they communicate with the 

 anterior diaphragmatic bronchus ; and by their base they send out a prolongation which 

 conducts the air into all the vertebrae of the neck and back, into all the vertebral ribs, 

 and, finally, into the spinal canal. 



" In their cervical portion, these prolongations present themselves in the form of two 

 canals extending from the base of the cervical reservoirs to the base of the cranium, 

 where they terminate; parallel and contiguous to the vertebral arteries, like them 

 they are lodged in the canals excavated in the substance of the transverse processes. 



" From their external part arises, at the six last cervical vertebrae, as many diverticuli, 

 which, lying against each other, pass from each side in the muscles of the neck, sur- 

 rounded by a common fibrous envelope, and apparently form a kind of canal at the 

 inferior part of this region ; when, however, this fibrous membrane is removed, it 

 becomes easy to isolate them, and it is then seen that they are completely independent, 

 and resemble small cornua. Highly developed in Palmipeds, they are only present in a 

 rudimentary state in the other classes. 



" On the internal side of these conduits, we see, at the level of each vertebra, one or 

 more orifices by which the air enters their interior ; and at the intervertebral foramina 

 another orifice, which allows it to pass into the spinal canal. From the communication 

 established by these orifices between the respiratory apparatus and the spinal canal, it 

 follows that in birds the cervical region is traversed by three atmospherical currents 

 two lateral or intertransverse, parallel to the vertebral arteries ; the third median or 

 interspinal, parallel to the spinal cord. 



" Just as the medullary tissue is replaced by air in the bones of birds, so might it be 

 imagined that the sub-arachnoidean fluid was also replaced by air around their spinal 

 cord ; and observation justifies the correctness of this prevision. The dura mater, whose 

 capacity is so superior to the volume of the marrow in Mammals, exactly measures the 

 volume of that organ in birds; so that there does not exist between the fibrous and 

 nervous surfaces any space for an accumulation of liquid ; this anatomical fact is 

 sufficient to demonstrate the absence of sub-arachnoidean fluid in birds. In denying the 

 existence of this fluid, it ought to be added that in this class of vertebrata, as in the 

 preceding, the spinal prolongation is covered by a triple envelope ; that in each, between 

 the pia mater and dura mater, is found a thin transparent membrane, which is lubri- 

 cated by a serous fluid ; but here this fluid does not collect, it only moistens the 

 arachnoid membrane. 



" Considered in their dorsal portion, the prolongations springing from the cervical 

 reservoirs offer an entirely different arrangement to that already noticed. The inter- 

 spinal current, having entered the thorax, terminates by passing into the first dorsal 

 vertebra ; after coursing through every part of this vertebra, it escapes by a lateral 

 orifice into a small sac situated between the two first ribs, at the origin of the first dorsal 

 nerve ; from this sac, it passes into the second vertebra by an opening placed on its 

 antero-lateral part, then it flows back from this into a new air-sac developed between 

 the second and third ribs ; and passing in the same manner into the third vertebra to 



