CESOPHAGUS, COMPARATIVE ANATOMY OF TIIE. In length. structure, and appearance in cross-section, there is the greatest variety in different groups of animals, and, indeed, an cesophagus may be entirely wanting, as, for ex ample. in Hydra. where the mouth opens directly into the digestive cavity. The coelenterates are divided into two great classes according to the origin of the (esophagus; in one, the Hydrozon, the (csophag,us. when present, has arisen from the endoderm, while in the other. the Scyphozoa, it has been formed by an invagination of the ecto derm. In all animals above ecelenterates, where a mouth is present, there is more or less of an (esophagus, but its appearance varies widely, with the form of the body. Long-necked animals have long wsophaguse* as a rule, while in ani mals with little or no neck the (esophagus may be very short. ln mammals its upper part is closely associated with the pharynx, and its muscles are striated (voluntary), but lower down the muscles are nnstriated (involuntary). In birds the (esophagus is very extensible and ca pable of holding a considerable amount of food in storage, and this ability is greatly increased by the development of a special pouch or pair of pouches on the ventral side of its lower part, known as the 'crop.' This crop may be simply
a sac for receiving the food or it may be an accessory digestive organ, the juices which it secretes serving to soften and macerate food stored in it. In pigeons during the breeding sea son this secretion becomes very abundant and milky in character, and is regurgitated into the mouths of the young along with macerated grain. Such fond is popularly called 'pigeon's milk.' Regurgitation is very easy for all birds and is habitual with many as a means of defense, or (by lightening their weight) of escape. In both bird; and mammals the (esophagus is a musculo membranous tube, made up of an outer layer of longitudinal (contractor) muscle-fibres, then a layer of circular (constrictor) musele-fibres, then loose connective tissue containing a thin layer of longitudinal fibres, and finally the glandular mucous membrane, which forms a lining, and is generally folded and plaited, often papillose, un less fully distended.