116 



ANIMAL KINGDOM. 



greater than in the amphibia. Their bones are 

 more consolidated than in the lower vertebrata, 

 their pel vie arch, when developed, is more firmly 

 attached to the vertebral column, the centres of 

 ossification, especially of the cranial bones, 

 generally remain detached, the extremities are 

 for the most part more competely developed, 

 and the toes are generally provided with claws. 

 Their cerebellum is remarkably small, their 

 muscular irritability languid, and they have 

 great tenacity of life. This ventricle, which 

 receives both the venous and arterialised blood, 

 is more or less divided by an ascending imperfect 

 septum. The thoracic and abdominal cavities 

 are not separated by a muscular diaphragm, and 

 the lungs extend backwards over the abdominal 

 viscera. Their organs of generation are double 

 in both sexes, and symmetrically developed on 

 the two sides of the body. The two portions 

 of the corpus cavernosum are often detached 

 and bifid ; the chorion of the ova is generally 

 thin or coriaceous, seldom calcified or hard, 

 and the instincts of the parent generally extend 

 to the protection of the young. (See REPTILIA.) 

 22. Aves, warm and red-blooded, ovipa- 

 rous, vertebrated animals, with four cavities of 

 the heart, covered with feathers, and with their 

 arms organized for flight. Their bones are the 

 most compact and dense in texture, the most 

 extensively anchylosed, and generally contain 

 air admitted from the cells of the lungs. Their 

 tympanic bone is moveable, they have horny 

 mandibles in place of teeth, their coracoid 

 bones reach the sternum, the sternal ribs are 

 ossified, and they want the tarsal bones. Their 

 diaphragm never forms a complete partition 

 between the thoracic and abdominal cavities. 

 The hemispheres of the brain are without con- 

 volutions, the optic lobes are large and hollow, 

 the cerebellum is large and sulcated, and the 

 posterior enlargement of the spinal chord of 

 great size. The great irritability of their mus- 

 cular system corresponds with the great extent 

 of their respiration, the high development of 

 their nervous system, the rapidity of their cir- 

 culation, and the increased energy of all their 

 functions. Their alimentary canal is furnished 

 with a crop, a glandular infundibulum, a giz- 

 zard, and generally with two cceca-coli, as seen 

 in the annexed diagram (Jig. 47), showing the 



Fig. 47. 



common form of these parts in a gallinaceous 

 bird. In these gallinaceous birds the oesopha- 

 gus (a) sends out at a right angle with its 

 course a large crop (6), with a contracted 

 neck, and supplied with glandular follicles. 

 Beneath this is the infundibulum or glandular 

 stomach (c), with numerous large follicles 

 placed between the mucous and muscular 

 coats, and this opens into the large muscular 

 gizzard (d*), provided externally with two strong 

 digastric muscles (e}. The cardiac and py- 

 loric orifices of the gizzard are close to each 

 other (/), and towards the lower part of the 

 small intestine a minute coecum often indi- 

 cates the original entrance of the yolk-bag. 

 The two long cceca-coli (g) commence by nar- 

 row entrances (A), and the short colon ends 

 in a common cloaca (/) for the genital arid 

 urinary secretions. 



Inthepredaceous birds, as the eagles (figAQ), 

 the oesophagus (), the crop (6), the infundibu- 

 lum (c), and the gizzard (de\ are capacious, thin, 



Fig. 48. 



and membranous, and form a continuous cavity 

 for the prey, from which the indigestible parts 

 can be thrown out in a bolus. In these birds 

 the cceca-coli (g) are very small, sometimes 

 unequal, or wanting. The urinary () and 

 genital organs (kk) enter the cloaca (/) near 

 the anus. The right ventricle of birds has the 

 tricuspid valve in form of a thick strong mus- 

 cular fold, and the aorta descends on the right 

 side. The lungs form two undivided, light- 

 coloured lobes, fixed by pleurae to the back part 

 of the trunk, the last rings of the trachea form 

 an inferior larynx, the bronchi pass in a mem- 

 branous form through the lungs, and the lungs 

 open into large membranous abdominal air- 

 cells, which communicate with the interior of 

 the bones. This extensive aeration of their 

 systemic as well as their pulmonic vessels gives 

 energy to their muscles for their aerial life and 

 their distant migrations, and a high tempera- 

 ture to their body for the incubation of the 

 egg. Their plumage and their downy covering 

 are the best suited for their aerial life and their 

 high internal heat. Their organs of generation 

 are double and symmetrical in the male, and 



