THE HEAD AND NECK. in 



that specially qualify the trachea for its important duty. The front part of the 

 trachea is round, the back part flat. The space between the ends of the imper- 

 fect rings of cartilage behind is filled by the musculus trachealis. This consists 

 of transverse and longitudinal unstriped fibres. The arrangement of these is 

 such as to set up a vermicular movement in the trachea when expectoration 

 occurs. 



The nerve-supply of the trachea comes from the vagus and also from the 

 recurrent laryngeal, a branch of the vagus, as this latter nerve lies in the fatty 

 groove between the trachea and oesophagus ; and from the sympathetic, which 

 latter nerves accompany the tracheal arteries, and take the same name. 



The blood-supply of the trachea comes from the inferior thyroid artery, a 

 branch of the thyroid axis of the subclavian artery. (Fig. 36.) The veins join 

 the thyroid plexus of veins. Of the thyroid veins, you will remember, the right 

 opens into the right innominate, the left into the left innominate vein. Each of 

 these has valves where it becomes tributary to the larger vessel. These thyroid 

 veins communicate on the trachea. These are the troublesome vessels in low 

 tracheotomy. 



Relations of the Trachea. If you will review your dissection, you will find in 

 front of the cervical part of the trachea the isthmus of the thyroid gland (Fig. 27), 

 the inferior thyroid veins, the sterno-hyoid and sterno-thyroid muscles (Fig. 27), 

 the deep cervical fascia, superficial fascia, and skin. Behind, the trachea is in rela- 

 tion with the oesophagus. On each side are the common carotid arteries, the infe- 

 rior thyroid artery, the recurrent laryngeal nerve, and the lobes of the thyroid 

 gland. (Fig. 31.) The thoracic part of the trachea, as you will see at a later 

 stage of your work, is covered by the manubrium sterni, the remains of the thymus 

 gland, the left brachio-cephalic vein, the aortic arch, the innominate and left com- 

 mon carotid arteries, and the deep cardiac plexus. Posteriorly is the oesophagus, 

 and laterally the vagus nerves are on each side. 



THE NASAL FOSS.E INTERNAL NOSE. 



Dissection. With a sharp saw cut through (i) the symphysis of the mandible. 

 Then take a sharp knife and cut through the tongue from base to tip in the mid- 

 line, as far forward and downward as the hyoid bone. (2) Let an assistant hold 

 the divided halves of the mandible apart, while you saw through the mid-line of 

 the remainder of the face. (Fig. 64.) Precaution : Before you make the latter 

 cut, observe to which side the septum nasi is deflected ; cut on the opposite side, 

 so as to have the septum intact for study. The cuts have placed before you for 

 study the mouth and nasal fossae. Very little dissection in addition to that already 

 done is necessary. 



The nasal fossae have the following geometrical parts : 



1 . Roof, formed by the nasal bones, the nasal spine of the frontal bone, crib- 

 riform plate of the ethmoid, the under surface of the body of the sphenoid. 



2. A floor (Fig. 74), formed by the hard and soft palate, covered by mucous 

 membrane. The hard palate is formed by the palatine process of the superior 

 maxilla and the horizontal part of the palate bone. 



3. An inner wall (Fig. 74), formed by the septum nasi, covered by mucous 

 membrane. The septum nasi is composed of (i) thevomer ; (2) the vertical plate 

 of the ethmoid ; (3) the cartilaginous septum quadrangular in some, triangular 

 in other cases. 



The outer surface is formed by the nasal bones, the nasal process of the 



