58 ANIMAL LIFE 



The pointed head serves to form a passage through 

 the air, into which the rising slope of the body follows, 

 whilst the falling slope of the tail allows the air to 

 glide off rapidly. The weight is taken off all the 

 peripheral parts and centred in the massive muscles 

 of the breast and the parts adjoining ; yet so delicately 

 poised that by extending its mobile neck the bird, 

 gliding with outstretched wings, descends where it will. 

 By letting the legs fall the head is thrown upwards, 

 the point of balance being thus moved slightly for- 

 wards or backwards without the aid of the wings. 

 These wings or sails are essentially fore-limbs, similar 

 to our own, but modified in every feature for the 

 needs of flight. The arm rises upwards and back- 

 wards, the forearm forwards and upwards, the hand 

 backwards and upwards, forming at rest a zigzag, 

 with the elbow pointing backwards and the wrist 

 forwards, but in action extending into a straight line, 



Fig. io. — The Pigeon. Dissection of the male from the right side. Half 

 the liver and the greater part of the intestine have been removed. In 

 the right wing, the bones and the arrangement of the primary and 

 secondary feathers are shown. — (From Marsha// and Hurst, ' Practical 

 Zoology. ') 



A, nostril. AD, ad-digital primary feather. B, external auditory meatus. BW, 

 bastard wing. C, 'esophagus. CA, right carotid artery. D, crop. DA, aorta. E, keel 

 of the sternum. F, right auricle. G, right ventricle. HV, hepatic vein. Hi, left bile- 

 duct. H2, right bile-duct. 1, distal end of stomach. IA, right innominate artery. 



IV, posterior vena cava. JA, left innominate artery. J V, right jugular vein. K, gizzard. 

 L, liver. M, proximal limb of duodenum. MD, the two mid-digital primary feathers. 

 MP the six metacarpal primary feathers. Mi, first metacarpal. M2, second metacarpal. 

 M 3, third metacarpal. N, cloacal aperture. Ni, first digit of manus. O, bursa Fabricii. 

 Oi, proximal phalanx of second digit of manus. O2, distal phalanx of second digit of 

 manus. P, pancreas. PA, right pectoral artery. PD, pre-digital primary feather. 

 PV, portal vein. Pi, first pancreatic duct. P2, second pancreatic duct. P3, third 

 pancreatic duct. Q, pygostyle. R, rectum. RC, radial carpal bone. RX, rectrices or 

 tail-feathers. Ri, third digit of manus. S, ureter. SA, right subclavian artery SV, 

 right anterior vena cava. T, rectal diverticulum. U, kidney. UC. ulnar carpal bone. 



V, pelvis. W, lung. X, humerus. Y, radius. Z, ulna. 



