150 THE ANATOMY OF THE HORSE. 



the trachea, they do not form complete circles, but have rather a 

 resemblance to the letter C with its ends overlapping. In consequence 

 of this configuration of the rings, the trachea is not circular on section, 

 but flattened in the vertical direction ; and the overlapping of the 

 extremities of the cartilages takes place on the middle of the upper 

 aspect of the tube. The breadth of the rings is not quite uniform, but 

 averages about half an inch. Here and there, however, two adjacent 

 rings may be more or less fused by the obliteration of the uniting 

 fibro-elastic membrane. In the thoracic portion of the tube the 

 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 overlapping 

 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 3h<sde. — This is a layer of non-striped muscular tissue 

 having its fibres 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 Subimicous 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 lieing 

 ciliated. 



Structure of the CEsophagus. This comprises (1) a muscular coat, 

 arranged in two layers ; (2) a submucous coat; a:id (3) a mucous lining. 

 The Muscular Coat consists of (ct) 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 tlie heart, the nniscular 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 l^chind that point 

 the tube is therefore pale like the stomach or the intestines. 



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



