224 VHILOSOPtlY Ol' ZOOLOGY. 



nura, in the form of a long narrow conglomerated gland. 

 Its ducts are seldom united with the biliary ones. 



The spleen is small, and situate between the left lobe 

 of the liver and the stomach, in the immediate neighbour- 

 hood of the gastric zone. 



The circulating system is chiefly distinguished from thait 

 of quadrupeds, by the structure of the heart. The pul- 

 monic ventricle, instead of having a membranous valve 

 (such as is found in both ventricles of quadrupeds, and 

 also in the systemic ventricle of birds), is provided with a 

 strong, tense and nearly triangular muscle, which Blumen- 

 BACH considers as assisting in driving the blood with greater 

 force into the lungs *. 



The organs of respiration present several remarkable pe- 

 culiarities. The trachea is composed of cartilaginous or 

 bony rings, which are usually complete, especially towards 

 the upper extremity. The larynx is in the form of a simple 

 slit, Avith a cartilaginous margin. It is strengthened by 

 several thin plates of bone, which have not been very 

 accurately examined. At the division of the bronchiae, 

 there are membranous spaces, or osseous capsules, which 

 essentially contribute to the formation of the voice. These 

 osseous capsules are very obvious in the males of the goose- 

 anders and many ducks. Besides these peculiarities, the 

 trachea in many birds suffers one or more enlargements 

 in its course. In other cases, it is convoluted towards its 

 bronchial extremity, and the convolutions are either con- 

 tained in the cavity of the thorax, or received into a cham- 

 ber hollowed out in the breast-bone -f. 



• Comp. Anat. p. 241. 



•f The reader who is desirous of further information concerning the tra- 

 chea of birds, may consult " An account of some peculiar advantages in the 



