Characteristics of the Vertehrata. liii 



tortuouSj and dilated at intervals. Its cartilaginous supports form 

 usually perfect ring's^ and are not rarely ossified. In most Birds 

 with the exception of the Hatltae, a lower larynx is developed upon 

 the junction of the trachea with the two bronchi. The bronchi 

 lose their cartilag-inous rings when they enter the lungs, where they 

 dilate into membranous canals which subsequently become smaller 

 by giving off branches, and finally end by opening into air sacs. 

 The lungs are deeply indented in correspondence with the ribs, but 

 are not otherwise lobed. There are nine air sacs, of which one is 

 placed asymmetrically between the furculum and the trachea, two 

 in the abdomen and pelvis, four in the posterior and lateral parts of 

 the thorax, and one on either side of the azygos interclavicular sac. 

 Processes are prolonged from the anterior and posterior of these 

 sacs into the bones. The bones of the skull are sometimes, as in 

 Mammals, the only pneumatic bones \ the vertebrae, humerus and 

 sternum come next in order as to the possession of air cavities; 

 whilst in some Birds all the bones of the body are said to have been 

 observed to be pneumatic. The bones of the fore-arm, on the other 

 hand, of the lower leg, of the manus and of the foot, are often found 

 to retain their medulla, and to be devoid of pneumatic cavities. 



The kidneys are divided into three lobes, and have their outlines 

 conformed to the sinuosities of the pelvic bones. There is never 

 any urinary bladder ; the ureters open internally to the generative 

 ducts, either directly into a cloaca, or into a urogenital pouch of 

 small autero-posterior extent. 



The brain is much larger relatively both to the entire body and 

 to the spinal cord than it is in Reptiles, and the spinal cord again 

 holds a more favoui'able relation to the entire body than in those 

 cold-blooded creatures. It occupies however a greater relative 

 length in the spinal canal than it does in Mammals, and in this 

 resembles the cord of the other Sauropsida. The cerebellum has 

 only rudimentary lateral lobes ; its grey matter however is con- 

 siderable in quantity, owing to its transverse lamination. It 

 projects forwards so as to come into relation with the prosen- 

 cephalon, and, as it were, to displace the bigeminal hollow optic or 

 mesencephalic lobes on to either side of its forward prolongation. 

 The cerebral hemispheres are represented by a thin shell of nervous 

 matter, which covers the large corpora striata, and incloses the 

 lateral ventricles. The corpus callosum is absent ; and the fornix 



