532 ZOOLOGY. 



Between the liver and the glandular stomach lies the 

 small, somewhat elongated, reddish brown spleen. 



In birds, as in most vertebrates, several spinal nerves unite 

 to form a brachial plexus, part of which is shown at B, and 

 which supplies the wings. Posteriorly, there is also formed 

 a plexus, the lumbar, for the legs. 



The muscles of tiie limbs are much modified in accordance 

 with the peculiar locomotion of birds. In connection with 

 the power of flight, the sternum has a very large keel, to 

 which are attached the pectoral muscles. T\\e pectoralis 

 major (Pe) is the most external ; it arises from the outer 

 half of the keel and is inserted into the humerus, and effects 

 the downward stroke of the wing. The second pectoral 

 {pectoralis tertius of some authors and the homologue of 

 the comparatively insignificant subclavius of human anat- 

 omy) arises from the inner portion of the keel, runs forward 

 and outward, and, tapering off, passes through a groove be- 

 tween the coracoid and sternum, as over a pulley, to be in- 

 serted into the humerus. The wing is raised by its action. 

 In the ostrich, etc. {Ratitm), the breast-bone has no keel, 

 and the disposition of the muscles of the rudimentary wings 

 therefore differs greatly from that here described. (Minot.) 



The ovary may be distinguished by the large incipient eggs 

 forming the greater part of the mass. The right ovary is 

 usually undeveloped, but when partly formed, as in some 

 hawks, the eggs do not mature. 



The " white" is deposited around the true egg in the upper 

 part of the oviduct, while the shell is secreted from glands 

 emptying into the lower part of the duct. The eggs of 

 birds are enormous in proportion to those of other verte- 

 brate animals, except the lizards. The egg of the ^-pyornis, 

 an extinct bird of Madagascar, is about a third of a metre 

 (13^ inches) in length, and as the egg is in reality a cell, 

 this is the largest cell known. The development of the 

 chick is better known than that of any other animal. It 

 travels the same developmental path as other vertebrates in 

 which an amnion and allantois are formed. About the sixth 

 day of embryonic life the bird-characters begin to appear, 

 the wings begin to differ from the legs, the crop and giz- 



