DISSECTION OF THE HEAD AND NECK. 149 



the medulla oblongata. It leaves the cranium by the posterior part of 

 the foramen lacerum, and inclining downwards and backwards on the 

 guttural pouch, it meets the cervical cord of the sympathetic, with 

 which it becomes in nearly all cases closely united. The common cord 

 resulting from the fusion of the two nerves descends in company with 

 the carotid artery, lying on the upper side of that vessel. At the lower 

 pai't of the neck the two nerves, in passing into the thoi-ax, again 

 become separate. 



The cervical cord of the sympathetic begins at the superior cervical 

 ganglion, which rests on the guttural pouch. After a short course it 

 unites, as just described, with the vagus. 



No branches are given off from either the vagus or the sympathetic in 

 the part where they form a common cord. 



The Inferior Laryngeal (Recurrent) Nerve (Plate 28). This 

 is a branch of the vagus, given off" within the thorax. The right nerve 

 has its origin in front of the heart, and is reflected round the dorso- 

 cervical artery. The left nerve is longer than the right, having its 

 point of detachment at the base of the heart, where it is reflected round 

 the root of the posterior aorta. The nerves pass forwards on the trachea, 

 and enter the neck by passing between the first pair of ribs. In the 

 neck each nerve ascends below the carotid artery, the right nerve 

 resting on the trachea, but that of the left side being, for the greater 

 part of its course, on the oesophagus. The nerves will subsequently 

 be followed in their distribution to the larynx. In the neck each 

 recurrent nerv^e throws off" branches to the trachea and oesophagus. 



Directions. — At this stage the dissector of the fore limb will be 

 engaged with the dissection of the axilla, and the dissector of the neck 

 should co-operate with him in the examination of the mode of formation 

 of the brachial plexus of nerves, and, thereafter, of the levator anguli 

 scapulee muscle. For the brachial plexus turn to page 3, and for the 

 levator anguli scapulae to page 8. 



Prepectoral Lymphatic Glands. This is a large group of glands 

 placed beneath and at the side of the great vessels at the entrance to 

 the chest. They are placed on the course of the lymphatic vessels of 

 the head, neck, and fore limb. 



Directions. — A segment about six inches in length may now be cut 

 from the trachea, and a similar segment from the oesophagus. These 

 are to be dissected to display the structure of the two tubes. 



Structure of the Trachea. This comprises (1) a framework of 

 cartilages united by (2) fibro-elastic membrane ; (3) an incomplete 

 layer of non-striped muscular tissue ; (4) a submucous layer ; and (5) 

 a mucous lining. 



The Cartilages of the trachea number between fifty and sixty, and 

 are of the hyaline variety. Although usually denominated the rings of 



