154 THE ANATOMY* OF THE HORSE. 



extremities of the rings do not meet, and the deficiency is there made 

 up by a number of thin cartilaginous pieces of irregular size and 

 shape, and somewhat imbricated in their arrangement. The rings are 

 thickest and strongest in their central portion, and thinnest at their 

 extremities. 



The Fibro-elastic Membrane. — This connects the adjacent edges of the 

 cartilages, and at the upper wall of the tube it connects their over- 

 lapping extremities. Its extensibility and elasticity permit the length 

 of the trachea to be accommodated to the movements of the neck, 

 and these properties will be made very evident by alternately extending 

 and relaxing a segment of the tube containing five or six rings. 



The Trachealis Muscle. — This is a layer of non-striped muscular tissue 

 of which the fibres are directed transversely. It does not extend all 

 round the tube, but is confined to its upper part, where the fibres lie 

 internal to the extremities of the rings or the fibro-elastic membrane. 

 The fibres form a continuous band, being not only placed under each 

 ring, but also in the interval between adjacent rings. 



The Submucous Coat is composed of areolar connective-tissue with 

 numerous elastic fibres longitudinally disposed. It also contains many 

 small compound racemose glands, whose mucous secretion is discharged 

 by ducts opening on the free surface of the mucous membrane. 



The Mucous Membrane, which forms a complete internal lining to 

 the tube, possesses a stratified epithelium, the surface layer of cells 

 being ciliated. 



Structure of the (Esophagus. This comprises (1) a muscular 

 coat, arranged in two layers; (2) a submucous coat; and (3) a mucous 

 lining. 



The Muscular Coat consists of (a) an outer layer of fibres longi- 

 tudinally disposed, and (b) a deeper layer in which the fibres are 

 arranged as transverse or oblique rings. In the cervical part of the 

 tube, and in the thoracic part about as far as the heart, the muscular 

 fibres are for the most part of the striped variety, and the tube has 

 there the external appearance of a voluntary muscle. About the 

 centre of the thorax, however, the character of the fibres gradually 

 changes to the pale, non-striped variety of muscular tissue, and 

 behind that point the tube is therefore pale like the stomach or the 

 intestines. 



The Submucous Coat is composed of areolar connective-tissue contain- 

 ing the alveoli of numerous mucous glands, whose ducts penetrate the 

 mucous membrane. It forms a very loose bond of connection between 

 the muscular and mucous layers ; and when the oesophagus is cut 

 across the mucous coat appears almost as if it lay independently within 

 the muscular layer. 



The Mucous Membrane has a thick stratified epithelium; and, except 



