98 THE BODY AT WORK 



yolk-sac, but has no openings on the exterior until it joins 

 up with two epiblastic pits one the stomodseum, or mouth- 

 cavity, at the anterior end ; and the other the proctodaeum, 

 at the posterior end of the body. The distinction between the 

 middle closed portion of the alimentary canal and its two secon- 

 dary openings suggests morphological speculations, into which 

 we have not space to enter, as to the ancestry of the verte- 

 brates. The majority of anatomists believe that the primitive 

 canal is represented in the middle portion, and that, in pre- 

 vertebrate animals, it opened to the exterior in a different way. 

 The pharynx is 4J inches long. It is enclosed by three thin 

 muscles, which overlap from below upwards the constrictors 

 of the pharynx. The anterior attachment of the superior 

 constrictor is to the jaw; of the middle constrictor to the 

 hyoid bone ; of the inferior constrictor to the thyroid cartilage. 

 Above the soft palate the nasal chambers communicate with 

 the pharynx by the posterior nares. Below the hyoid bone, 

 which is easily felt in the neck as a bony arch just above the 

 thyroid cartilage (Adam's apple), the windpipe, or trachea, 

 joins the pharynx by a single pear-shaped orifice the rima 

 glottidis. When we consider the mechanism of swallowing, we 

 shall study the arrangements which prevent food, passed 

 through the fauces, from entering either the nasal chambers 

 above or the windpipe below and in front. At the level of 

 the lower border of the thyroid cartilage the pharynx becomes 

 the relatively narrow oesophagus. This tube, which lies 

 behind the trachea, and slightly to its left side, passes with a 

 straight course to the abdomen. It traverses the chest, lying 

 behind the heart, pierces the diaphragm, and just beneath it 

 joins the stomach. Its length is about 9 inches. The stomach is 

 a sickle-shaped bag. It has two apertures the cardiac orifice, 

 or junction with the oesophagus ; and the pyloric orifice, or 

 junction with the small intestine. It is so folded on itself that 

 these two apertures are not more than 4 inches apart. Its 

 outline may be drawn on the body-wall with a piece of char- 

 coal from a point an inch below and an inch to the left side of the 

 lower end of the breast-bone, the position of the cardiac orifice, 

 to a point about 4 inches below the end of the breast-bone, 

 and an inch or two to the right side of the midline of the body, 

 the position of the pyloric orifice, with a slight curvature to 



