This orifice is dilated during inspiration, by the contraction of the 

 two first fasciculi of the diaphragm. 



The Cervical Air-sacs. — The two cervical air-sacs are located just 

 above the thoracic air-sac at the inferior part of the neck and in 

 front of the lungs. They are cone-shaped with the base directed 

 forward and the apex backward. They are related superiorly with 

 the cervical muscles, and inferiorly with the thoracic air-sac from 

 which they are separated by the trachea, the esophagus, the pneu- 

 mogastric nerve, and the jugular veins. The walls touch each 

 other internally, and form a median septum which includes in 

 its substance the two common carotid arteries. Externally they 

 are related to the origin of the cervical nerves, to each of which they 

 contribute a small sheath. They surround the vertebral artery, 

 and are connected with the subcutaneous muscles and the skin. 

 The summits communicate with the anterior diaphragmatic bron- 

 chus. Prolongations extend from their bases which conduct the air 

 into all the vertebras of the neck and the back, into all the vertebral 

 ribs, and into the spinal canal. Parallel with and adjacent to the 

 vertebral arteries, and lodged in the canals excavated in the trans- 

 verse processes of the cervical vertebrae, are two cervical prolonga- 

 tions, one on each side, which extend to the cranium from the base 

 of the cervical reservoirs. From their sides, at the last six cervical 

 vertebra;, are six extensions in the form of diverticuli, which, 

 lying against each other, pass from each side into the muscles of the 

 neck. They are surrounded by a thin fibrous envelope, a continua- 

 tion of the mucous l inin g of the sac, and apparently form a canal in 

 the inferior part of this region. These prolongations are better 

 developed in palmipedes than in chickens. On the internal side 

 of these prolongations, one or more foramina penetrate the vertebral 

 segment, which allow the extensions of the prolongations into the 

 spinal canal. Chauveau states that "as the medullary tissue is 

 replaced by air in the bones of birds, so is the subarachnoid fluid 

 replaced by air aroimd the spinal cord." 



The prolongations extending from the cervical air-sacs, having 

 entered the thorax, terminate by passing into the first dorsal verte- 

 bra. After permeating every part of this vertebra, it escapes by a 

 lateral opening and forms a small sac located between the first two 

 ribs, near the origin of the first dorsal nerve. From this sac an 



