DISSECTION OF THE DOG 149 



A. carotis communis. — The frn o common carotid arteries leave the brachio- 

 cephalic at no very great distance from each other. The right artery runs 

 obliquely laterahvards across the ventral surface of the trachea, and then 

 along the lateral face of this tube. The left artery has a similar relation to 

 the trachea about the thoracic inlet, but in the neck it is in contact with the 

 oesophagus. Each artery is in relation dorso-laterally with the combined 

 trunk of the vagus and sympathetic nerves. 



On a level with the wing of the atlas the common carotid artery divides 

 into external and internal carotid and occipital arteries. The collateral 

 branches of the artery are : (1) The caudal thyroid artery (a. thyreoidea caudalis), 

 a small vessel arising at the entrance to the chest and accompanying the 

 recurrent nerve up the neck to end in the thyroid gland ; (2) the cranial 

 thyroid artery (a. thyreoidea cranialis), an artery of some size suj^plying the 

 thyroid gland and contributing the ascending pharyngeal artery (a. pharyngea 

 ascendens), muscular (rami musculares) and glandular (rami glandulares to 

 the submaxillary gland) branches ; (3) the laryngeal artery (a. laryngea), a 

 small vessel ramifying in the larynx. 



N. vagus et truncus sympathicus. — The thick nerve cord lying dorsal 

 and lateral to the common carotid artery is formed on a level with the wing 

 of the atlas by the union of the vagus nerve and the cervical cord of the 

 sympathetic. These are bound together by strong connective tissue. 



N. recurrens. — The recurrent nerve, a branch of the vagus already 

 encountered in the chest, follows the lateral aspect of the trachea and ends in 

 the larynx. In the neck the nerve supplies branches to the trachea and oeso- 

 phagus (rami tracheales et cesophagei). 



M. mylo-hyoideus. — Lying in the space between the two halves of the 

 mandible, the mylo-hyoid is a flat, broad, thin muscle composed of transverse 

 fibres arising from the ' mylo-hyoid line close to the alveolar border of the 

 mandible. The fibres join a raphe in the middle line, as well as the symphysis 

 of the mandible and the body of the hyoid bone. 



Dissection. — Turn the manubrium of the sternum, with the muscles attached 

 thereto, as far as possible towards the head, and so expose the trachea 

 and oesophagus. 



Trachea. — The trachea is an almost cylindrical tube extending from the 

 larynx to a point opposite the fourth rib, where it divides into the two bronchi. 

 The lumen of the tube is not quite uniform on account of a certain amount of 

 narrowing from the larynx to the thoracic entrance. Nor is the trachea abso- 

 lutely in the middle line since there is some degree of inclination towards the 

 right. At its commencement the Avindpipe is in contact dorsally with the 

 oesophagus, but later this relation is exchanged for one with the longus colli 

 muscle. 



The skeleton of the trachea consists of a variable number (thirty-five to forty) 



l.3 



