THE RESPIRATORY SYSTEM. 39 



the paired thyroid near the origin of the carotids; and the embryonic paired 

 thymus in the cervical region in the young of all birds. These ducts of the lym- 

 phatic system discharge by the ductus thoi'acicus into the superior vena; cuvse. 

 Communication is also common with the veins of the pelvic region. Certain 

 Ostrich forms, some waders, and various natatorial birds possess ''lymph 

 hearts" upon either side of the pygostylo. In other forms they are merely 

 non-contractile, vesicular enlargements. 



THE RESPJBATOKY SYSTEM. 



Anatomically, the resjnratory system in birds is far more interesting than 

 it is in mammals. There being, as a rule, only a rudimentary diaphragm, no 

 muscular partition separates the thoracic from the abdominal cavities, and the 

 lungs, thus unconfined, may extend between the root of the neck to the region of 

 the kidney in the pelvic basin, as they often do. They are unlobed, being situated 

 at the roof of the inner aspect of the thoraco-abdominal cavity, and covered 

 only upon their ventral side with the pleura, while upon their dorsal surfaces, 

 there being a pair of them, the ribs and vertebra; make their impress. The pul- 

 monary vessels, the bronchi, the nerves, and surrounding connective tissue, 

 form the roots of the lungs in birds. Associated with the respiratory system 

 we find a unique auxilary system in this group of vertebrates, known as the 

 avine air-sacs and air-spaces. These are not present in any other class of 

 animals, and in birds they vary greatly as to the extent of their presence. They 

 are developed to a most extraordinary degree in the North American Black 

 Vulture, ( Catharista atrata) in which species the present writer has studied 

 them. All the bones of the skeleton in this bird are hollow and air passes 

 freely through them by the action of the lungs in respiration. There are also 

 nine, more or less, independent air-sacs or cavities, subject to general or par- 

 tial inflation at the will of the bird, giving it the power to almost completely 

 permeate itself with air. Much has been written about the function of these air 

 sacs, and to say the least of it they offer wonderfully interesting studies for the 

 student of avian anatomy and physiology. 



Quite as interesting are the air-passages in birds, and we find the trachea 

 and its associated parts to present a great deal that is strictly unique in the 

 vertebrate subkingdom. It is fortified by a series of osseous rings of various 

 forms in different groups; it is attached to the sternum by a pair of 

 sterno-tracheal muscles; it may develop various remarkable enlargements at 

 its lower end, or near its middle; it may form a loop over the pectoral muscle 

 [Ortalis), or be coiled up in compartments intended to receive it in the body 

 and carina of the sternum ; or it may present other peculiar features. Quite 

 as curious is the morphology of the syrinx at its lower end and the lai^nx at 



