562 RESPIRATORY APPARATUS IN BIRDS. 



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 also why the other aerial sacs 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. 337, 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 

 correspond 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 f onn 

 of two canals extending from the base of the cervical reservoire 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, suiTounded 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 mter- 

 vertebral 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 ail/Cries ; the third median of 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 



I 



