HEAD AND NECK OF THE HORSE 9 



The cervical part (pars cervicalis) is at first in the middle line and 

 immediately dorsal to the trachea, but an inclination towards the left 

 soon becomes noticeable, and in the greater part of the neck the gullet 

 is wholly to the left of the median plane. From the fourth or fifth 

 cervical vertebra onwards it is directly to the left of the trachea. 



At its origin, the oesophagus is related dorsally to the diverticulum 

 of the auditory tube of Eustachius and the longus capitis and longus 

 colli muscles. Ventrally it lies on the cricoid cartilage and the dorsal 

 crico-arytenoid muscles; while laterally it is in contact with the 

 common carotid artery and its associated nerves. On a level with the 

 fifth cervical vertebra the gullet is dorsally related to the longus colli 

 muscle, is in contact with the trachea medially, and has the left common 

 carotid artery, the vagus, sympathetic, and recurrent nerves, and the 

 omo-hyoid muscle as lateral relations. In the vicinity of the entrance 

 to the chest, the left jugular vein lies along the lateral face of the 

 oesophagus. 



Some details of the structure of the oesophagus can be determined 

 with the naked eye. If a transverse section of the tube be made, it 

 will be evident that, like other tubular parts of the alimentary canal, 

 its wall is constructed in layers. On the outside is a loose, adventitious, 

 areolar covering (tunica adventitia) by which the oesophagus is connected 

 with the surrounding structures. The interior is lined by a thick, pale 

 mucous membrane (tunica mucosa), thrown into longitudinal folds when 

 the tube is not distended. The mucous membrane is attached to the 

 muscular layer of the wall by a loose submucous tissue (tela sub- 

 mucosa). 



The greater proportion of the thickness of the Avail is formed of 

 muscular tissue (tunica muscularis), red in colour in the fresh state,^ and 

 with fibres running in various directions. Though the arrangement of 

 the bundles of fibres is complicated, it is possible to distinguish three 

 closely connected strata. On the outside there is a layer of longitudinal 

 fibres most clearly marked towards the pharyngeal end of tlie tube, 

 where they form bundles running along its lateral margins. Within the 

 longitudinal stratum are two layers of oblique or spiral fibres. 



It should be noted that at the commencement of the oesophagus two 

 divergent bundles of muscular fibres are continued on to it from the 

 region of the raphe of the pharynx ; these are interlaced with fibres 

 continued from the cricopharyngeal muscle. Two other small bundles 

 are also associated with the beginning of the oesophagus. These spring 

 from the cranial border of the cricoid cartilage and the depression 



1 The muscular tunic of the cesophagus consists of red, striated fibres as far as 

 where the tube crosses the base of the heart. 



