THE RESPIRATORY APPARATUS IN BIRDS. 475 



The thymus gland in young Ruminants is more voluminous than in the Foal, and is 

 situated higher up in the cervic;tl region. (In the Garnivora, it is divided into two branches ; 

 but it is small, and completely lodged between the layers of the anterior mediastinum. 

 It persists for some time after birth, and seldom disappears in less than a year.) 



f COMPARISON OP THE GLANDIFORM BODIES ANNEXED TO THE RESPIRATORY APPARATUS IN 

 MAN WITH THOSE OP ANIMALS. 



In Man the two lobes of the thyroid body are connected by an isthmus, and the upper 

 extremity of the lobe is carried up to the side of the thyroid cartilage. The isthmus 

 often gives origin to a process of variable length and size, called the pyramid or third 

 lobe, which is generally situated to the left A muscle is sometimes found connected 

 with the isthmus or pyramid, and is attached above to the body of the os hyoides or to 

 the thyroid cartilage ; it lias been named the levator glandulx thyroidx. 



The thymus gland is composed of two lobes, a right and left, only joined by 

 connective tissue, and having no structural communication. There is a cervical and a 

 thoracic portion, the whole extending from the fourth rib as high as the thyroid gland. 

 After birth it continues to enlarge until the end of the second year, and begins to 

 diminish between the eighth and twelfth years ) 



CHAPTER II. 



THE RESPIRATORY APPARATUS IN BIRDS. 



THE organs composing the respiratory apparatus of birds, offer conditions altogether 

 special, and which have a remarkable influence on the mechanism of respiration. The 

 modifications imposed upon the performance of this function will be indicated after an 

 examination of the tubular apparatus, which carries the air into the lung, and the 

 characters of that organ, as well as the air reservoirs (or sacs; annexed to it. 



THE TUBULAR APPARATUS. When this apparatus is compared with that of Mammals, 

 no very sensible differences are observed at least in domesticated birds. 



The nostrils, pierced through the upper mandible of the beak, have no membranous 

 and movable wings, and the na&al fossse open into the pharynx by a long, narrow si t 

 behind the bony palate. A transverse row of small, horny papillse, placed at the 

 anterior extremity of this aperture, represents the soft palate. 



The larynx has no epiglottis : a defect which does not prevent the complete occlusion 

 of the glottis during the passage of food, as the laryngeal orifice is ciicumscribed by two 

 lateral lips which then meet in the most exact manner. 



The trachea is composed of complete cartilaginous rings, and not simple arcs. In 

 song-birds, the last ring is a second larynx, the real organ which produces the modulated 

 voice of these creatures ; it only exists in a rudimentary condition in poultry, however, 

 the last tracheal piece in them being slightly dilated, and showing at the origin of the 

 bronchi a membranous layer, from whose vibration results cries or crowing. Other 

 singular peculiarities belonging to the trachea deserve to be described here, if they 

 were not the exclusive appanage of some wild fowl. We are content to mention the 

 presence of the bony drum found at the terminal extremity of the trachea in the 

 whistling duck, and the remarkable convolutions that tube forms in the breast-bone of 

 Cranes and male Swans. 



The bronchi only show incomplete rings in their structure. They pass into the lung 

 by its inferior face, towards the union of its anterior and two posterior thirds. When 

 describing this organ, their mode of ramification, and the nature of the relations they 

 bear to its proper tissue will be considered. 



THE LUNGS. M. Sappey, in the remarkable memoir published by him in 1847, has 

 described them as follows : " The lungs of birds are situated on the lateral parts of the 

 vertebras of the back which sepaiate them, and lying against the arch of the thoracic 

 cavity, to which they adhere. Their rosy colour resembles that presented by these 

 organs in Man and the Mammalia during uterine life, and for some time after birth; 

 they are especially remarkable for their restricted volume, which scarcely represents an 

 eighth part of the thoracic capacity. Their configuration is far removed Irorn the 

 conical form of the lungs in Mammals, and the oval torm of the same organs in reptiles ; 

 they are semi-elliptical, and if the two lungs of a Mammal were opposed base to base, 



