150 DISSECTION OF THE DOG 



of incomplete rings of cartilage (eartilagines tracheales) joined together by 

 membranous annular ligaments (ligamenta annularia). The interruption in 

 the continuity of each ring is placed at the dorsal part of the tube, where 

 transverse muscular fibres occur. The interior of the trachea is lined by a 

 mucous membrane. 



GEsophagtjs. — The oesophagus is that part of the alimentary canal which 

 intervenes between the pharynx and the stomach. For descriptive purposes 

 it is divided into cervical, thoracic, and abdominal parts (pars cervicalis, pars 

 thoracalis, et pars abdoniinalis) . In the earlier dissections the thoracic and 

 abdominal parts have been exposed and exa m ined. 



The cervical part is at first in the middle line and immediately dorsal to 

 the trachea, but an inclination towards the left soon becomes noticeable. At 

 the entrance to the chest the oesophagus may be at a level even ventral to that 

 of the trachea. 



Glandula thyreoidea. — One of the ductless glands, the thyroid consists 

 of two lateral lobes connected ventral to the trachea by an isthmus of variable 

 dimensions. Frequently the isthmus is absent ; but when present, it connects 

 the more caudal part of the two lateral lobes. Each lobe is elongated, with 

 narrow oral and aboral extremities, and lies on the lateral face of the trachea 

 immediately aboral to the larynx (over six or seven tracheal rings). The 

 lateral surface of each lobe is covered by the stemo-cephalic muscle, and its 

 ventral border is in contact with the sterno-thyroid muscle. 



In association with the thyroid gland are two or more small parathyroids. 



Dissection. — Remove the skin from the ear and the parotid region. Clean 

 up the auricular nerves, vessels, and muscles. 



Aueis externa. — Before commencing the dissection of the muscles of the 

 external ear, it is necessary to have some knowledge of the cartilages to which 

 they are attached. 



The cartilages of the external ear are three in number — conchal, scutiform, 

 and annular. The conchal cartilage or auricula is the largest, and forms the 

 projecting, conspicuous part of the ear. Its form varies greatly with the 

 breed of the dog ; but, generally speaking, it may be said to have a funnel-like or 

 trumpet-shaped outhne. The wide free part of the cartilage has two borders — 

 cranial and caudal — meeting at the tip of the ear. At the base of the cranial 

 border is an irregular projection known as the helix. At the lateral part of the 

 narrow, basal portion of the cartilage is a curved, plate-like projection, the 

 tragus ; and immediately caudal to this an irregular process, the anti-tragus. 



The scutiform cartilage is in the form of a plate of irregular outline, and lies 

 on the temporal muscle oral to the base of the conchal cartilage. 



The annular cartilage, as its name implies, is ring-like in form, and is adherent 

 to the margin of the external acoustic meatus of the temporal bone. 



