THE BESFlRATOnr APPARATUS IN BIRDS. 475 



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

 sitaated higher up in the oervioid region. (In the Carnivora, 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 birtU, and seldom disappears in less than a year.) 



'COMPABISON OP THE GLANBIFOEM BODIES ANNEXED TO THE BESPIRATOBT APPABATDS IN 

 MAN WITH THOSE OF 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 has been named the levator glandulm thyroidse. 



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 imtil the end of the second year, and begins to 

 diminish between the eighth and twelfth years.) 



CHAPTEE II. 



THE EESPIEATOEY APPABATDS 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 tuMlar apparatus, which carries the air into the [ung, and the 

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



The Tdbulab Apparatus.— When tliis apparatus is compared witli that of Mammals, 

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



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

 and movable wings, and the nasal fossffi open into the pharynx by a long, narrow sl.t 

 behind the bony palate. A transverse row of small, liomy papillae, 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 oiicumscribed by two 

 lateral lips which then meet in the most exact manner. , ^ - , r 



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

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

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

 the last tracheal piece in them being slightly dilated, and sliowing at the origin ol 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 

 we?e not the exclusive appanage of some wild fowl. We are content to mention the 

 presence of the bony drum found at tlie terminal extremity of the trachea iii the 

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



Cranes and male Swans. , . , , mu ■ t iv,„ i.,„„ 



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

 bv its inferior face, towards the union of its anterior and two posterior thu-ds. When 

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

 bear to its proper tissue will be considered. ,,.,,, ,. ■ loAr, x. 



The Lungs -M. Sappey, in tlie remarkable memoir published by him m 1847 has 

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

 vertebrae of the back-which sepriiate them, and lying ag.inst the arch of the thoracic 

 lavitv 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; 

 thfv are especially remarkaole for their restricted volume which scarcely represents an 

 eighth part of the thoracic capacity. Their oonhguratw^n is far removed Irom the 

 Sal form of the lungs in Mammals, and the oval torm of the same organs in ret-tiles ; 

 ttey are semi-elliptical%nd if the two lungs of a Mammal were opposed base to base, 



