438 



COLLEGE ZOOLOGY 



The forelimb or wing of the fowl is modi- 

 fied in many ways from that of the primitive 

 vertebrate. For example, the primitive verte- 

 brate had 5 digits numbered 1 to 5, begin- 

 ning with the thumb, whereas, the fowl has 

 but 3 digits (numbers 2, 3, and 4), and only 

 the number 3 digit is well developed. The 

 distal row of carpal bones and the 3 meta- 

 carpals are fused, forming a carpometacar- 

 pus; this adds to the rigidity of the wing. 

 The arm, as in other vertebrates, contains a 

 single bone, the humerus, with a convex 

 head which lies in the glenoid fossa. The 

 forearm possesses 2 bones, the radius and the 

 ulna. The wrist contains 2 carpal bones; 

 the other carpal bones are fused with the 3 

 metacarpals, forming the carpometacarpus, 

 as stated above. Besides the carpometacar- 

 pus, the hand possesses a second digit of 2 

 small bones, which supports a small tuft of 

 feathers and is known as the bastard wing 

 (alula); a middle digit with 3 phalanges; 

 and a fourth digit containing a single 

 phalanx. The main flight feathers or pri- 

 maries are supported by digits 3 and 4, the 

 secondaries by the ulna, and the tertiaries 

 by the humerus. 



The pelvic girdle consists of a pair of 

 ilia, the ischia, and the pubes, as in nearly 

 all of the vertebrates above the fishes. These 

 bones are firmly fused and united with the 

 posterior part of the vertebral column. 



The hindlimbs are used for bipedal loco- 

 motion. The thigh is concealed beneath the 

 feathers. The femur is the short thick thigh- 

 bone. In each leg there is a slender fibula, 

 and a long, stout tibiotarsus (drumstick), 

 the latter consists of the tibia fused with the 

 proximal row of tarsal bones. The ankle 

 joint is between the tibiotarsus and the 

 tarsometatarsus; the latter represents the 

 distal row of tarsal bones and the second, 

 third, fourth, and fifth metatarsals fused. 

 The foot possesses, besides the tarsometa- 

 tarsus, 4 digits; the first is directed backward 

 and is called the hallux; and the other 3 are 

 directed forward. Each digit bears a terminal 

 claw. The number of phalanges in each 



digit is one more than the digit number, 

 hence digit 1 has 2 phalanges, and digit 4 

 has 5. The tarsometatarsus of the do- 

 mestic fowl bears a backwardly directed 

 spur. 



Muscular system 



The muscles of the neck, tail, wings, and 

 legs are especially well developed. The mus- 

 cle that produces the downward stroke of 

 the wings, the pectoralis major, is the larg- 

 est; it weighs about Vs as much as the en- 

 tire body; it takes its origin from the sternal 

 keel and inserts on the humerus; the pec- 

 toralis minor muscle raises the wing. These 

 muscles constitute what is popularly known 

 as the breast of the bird. Connected with 

 the leg muscles is a perching mechanism 

 which enables the bird to maintain itself 

 upon a perch even while asleep. When the 

 bird squats down for resting or sleeping, a 

 pull is exerted on tendons which flexes all 

 of the toes and holds the bird firmly to its 

 perch. 



Digestive system 



Because of the high rate of metabolism 

 in birds, they require large quantities of 

 food, and digestion is rapid. Pigeons feed 

 principally upon vegetable food such as 

 seeds. The mouth cavity opens into the 

 esophagus (Fig. 306), which enlarges into 

 a crop; here the food is stored and moist- 

 ened. Pigeons are noted for their ability to 

 produce "pigeon milk," a cheesy secretion, 

 which results from the degeneration of cells 

 lining the crop, and is used for nourishing 

 their young. The stomach consists of two 

 parts, an anterior proventriculus with thick 

 glandular walls which secrete the gastric 

 juice, and a thick muscular gizzard (ven- 

 triculus), which grinds up the food with the 

 aid of grit swallowed by the bird. The slen- 

 der intestine with many coils leads to the 

 rectum at a point where two blind pouches, 

 the ceca, are given off. The digestive canal 



