THE LEFT DIVISION OF THE HEAD. 209 



have proceeded far ; and these divisions separate again and 

 again, until the tubes become very numerous, but very 

 minute, ending in very small cavities, termed the air-cells 

 of the lungs. 



The oesophagus {Plate VI. k) is the continuation of the 

 funnel-like cavity of the pharynx. It is a muscular mem- 

 braneous tube, leading from the pharynx to the stomach ; 

 it takes a course, inclining a little to the left side, and 

 to the posterior of the trachea, which it surmounts near 

 the head. It then penetrates the chest, and passes along 

 the dorsal vertebrae within the layers of pleurae ; and ulti- 

 mately pierces an opening in the diaphragm, to terminate 

 in the stomach. Its coats are, first, a copious cellular one. 

 Its second coat is muscular, and made up of two strata of 

 fibres, an external longitudinal and an internal circular. 

 The inner coat is cuticular and wrinkled into folds ; but 

 has upon it a distinct layer of longitudinal muscular 

 fibres, that it may adapt itself to the expansion or contrac- 

 tion of the tube : this cuticular portion extends over one- 

 half of the stomach. 



DESCRIPTION OF PLATE VII. 



THE LEFT DIVISION OF THE HEAD, SEPARATED BEFORE THE 

 MESIAN LINE IS ATTAINED. 



a, The superior turbinated bone, covered with the 

 Schneiderian membrane and its ramifying vessels ; b, the 

 inferior turbinated bone in the like condition ; c, the mouth 

 and gums of the lower jaw ; d, the facial and frontal 

 sinuses ; e, the ethmoid cells ; /, the ethmoid sinuses ; g, the 

 cavity of the skull, deprived of the brain and its mem- 

 branes ; h, the portion of the bony triquatrum which sepa- 

 rates the cerebrum and the cerebellum (see Plate VI.) ; i, 

 the bars upon the roof of the mouth ; j, the guttural pouch, 

 formed of merely mucous membrane, which shows the 

 various adjacent structures through its sides ; k, the open- 

 ing into the fauces of the guttural pouches ; I, the soft 

 palate (see/, also Plate VI.) ; m, a portion of the lacerated 

 submaxillary gland ; n, the opening of the nostril into the 

 fauces ; 0, the spinal canal ; p, a mass of confusion, 



p 



