INTRODUCTION. XI 



muscles, that proceed from the pelvis, pass over the knee 

 and the heel, and are inserted into the ends of the toes, so 

 that the simple weight of the body, bending the knee-joint 

 stretches the muscles and their tendons, and flexes the toes. 



The cutaneous muscles are strongly developed, and are 

 divided into distinct parcels. The great bulk and strength 

 of the pectoral muscles of most Birds has been already 

 alluded to. 



The digestion of Birds is in proportion to the energy of 

 their vitality and the amount of respiration. Salivary 

 glands exist in many, and in some, the edible-nest-produc- 

 ing Swifts, are developed enormously. The oesophagus, in 

 many birds, is dilated into a crop or craw, which in the 

 I*arrots and Pigeons, the most exclusively vegetable 

 feeders, is furnished with numerous glands, which become 

 developed in both sexes during the i^eriod of incubation, 

 and secrete a milky fluid with which the young are nourish- 

 ed at first. The craw is usually single, and on the right 

 side only, but in Pigeons it is double. In most birds it 

 simply serves as a reservoir for food hastily taken, and 

 which cannot be at once received into the stomach ; but the 

 grain is usually moistened there and softened. The sto- 

 mach of birds is double. The first, or proventriculus is a 

 membranous stomach furnished with a multitude of 

 glands, variously disposed and shaped in different groups, 

 the secretion from which softens and otherwise acts on 

 the aliment. In some birds, as the Grebes, there is a 

 contraction and intervening space between this and the 

 gizzard, and in one remarkable genus, Opisthocomus, this 

 is developed into a large cavity. In other birds, there is 

 no appreciable difference between the proventriculus and 

 gizzard, which is usually very muscular, and in many 

 (as in the Gallinace^e) enormously thick, being formed of 



