HEAD AND NECK OF THE HORSE 7 



at the entrance to the chest, where they are continuous with the 

 cranial mediastinal glands of the thorax. 



Associated with the cervical lymph glands there is a relatively large 

 lymphatic vessel (tracheal trunk) on each side of" the neck. This follows 

 the common carotid artery across the lateral face of the trachea, and, on 

 the right side of the body, can generally be followed to the caudal group 

 of cervical glands. On the left it ends by joining tlie thoracic duct as 

 this is entering the venous system. 



Dissection. — In order to obtain a clear view of the trachea, cut across 

 the common belly of the sterno-hyoid and sterno-thyroid muscles, and 

 turn these aside. 



The trachea. — The trachea^ is a cartilaginous and membranous 

 tube extending from the larynx (roughly from tlie level of the articula- 

 tion between the atlas and the epistropheus) down the middle line of the 

 neck and into the thorax, where it ends opposite the fifth or sixth inter- 

 costal space by dividing into the right and left bronchi. In the neck 

 the tube is slightly flattened dorso-ventrally, the dorso-ventral diameter, 

 in an animal of averasfe size, being about 5 cm., while the transverse 

 diameter is about 6 cm. The width of the tube, is, however, not quite 

 uniform, as there is a certain amount of narrowing in the neighbourhood 

 of the larynx as well as near the thoracic inlet. 



The main relations of the trachea in the neck are as follows. At 

 first the oesophagus is dorsal to the trachea, but later it passes towards 

 the left, and the trachea then comes into contact with the longus colli 

 muscle. On each side of the wdndpipe are the thyroid gland, the 

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

 the lymph glands of the neck and the lymphatic vessels connected 

 therewith, the omo-hyoid and scalene muscles, and, on the right side, 

 the jugular vein. Ventral to the trachea are the sterno-cephalic, sterno- 

 hyoid, and sterno-thyroid muscles. 



The skeleton of the trachea consists of a variable number (48 to 55) 

 of incomplete rings of cartilage (cartilagines tracheales) joined together 

 by membranous and elastic ligaments (ligamenta annularia trachealia). 

 The interruption in the continuity of each ring occurs in the dorsal part 

 of the tube, where transverse muscular fibres (m. trachealis) occur. The 

 first tracheal cartilage is joined to the cricoid cartilage of the larynx by 

 the crico-tracheal ligament (ligamentum cricotracheale), in series with, 

 but looser than, the annular ligaments. 



The dissector should isolate one or two of the tracheal cartilages, 

 when he will find that they are thickest (2-3mm.) and narrowest in the 



1 rpcLXi'La (tracheia) [Gr.], rough (artery). 



