ALIMENTARY CANAL. 209 



the vessels and nerves passing from the region of the vertebral 

 column to the viscera. By far the greater number of the nerves 

 arise from the sympathetic system ; cerebral and spinal elements 

 are present only in the most anterior and posterior sections of the 

 alimentary canal, both of which regions contain striped muscular 

 fibres and are under the influence of the will. The mesentery gives 

 rise to a large system of folds arising from the inner dorsal surface 

 of the body-wall, in which the viscera are enveloped. 



The most anterior section of the primitive alimentary tract 

 functions as a respiratory cavity as well as a food-passage, and 

 possesses for this purpose a row of apertures, lying one behind 

 the other :. round these, certain vessels are developed, by means of 

 which a continual interchange of gases can take place between 

 the blood and the water passing through the apertures. In short, 

 gills are developed (Fig. 171, A). Although these latter are only 

 functional in Fishes, Dipnoans, and aquatic (or larval) Amphibians, 

 even in the higher Vertebrates, the larger portion of the cavities 

 of the mouth and pharynx lying behind the internal nostrils serves 

 as a common air- and food-passage until a proper palate is formed 

 (Fig. 171, C, D). 



With the formation of a definite palate, 1 the primitive mouth- 

 cavity becomes divided into an upper respiratory, and a lower 

 nutritive portion, or into a nasal, and a secondary or de- 

 finitive mouth-cavity. The separation, however, is not a 

 complete one, the passage being common to both cavities for a 

 certain region (Fig. 171, at f). This region is called the pharynx, 

 and in Mammals it is partially separated from the mouth by a 

 fibrous and muscular fold, the velum palati, or free edge of the 

 soft palate. 



The alimentary canal of all Vertebrates is divided into the 

 following principal sections: Mouth (cavum or is), pharynx, 

 gullet (oesophagus), stornach(ventriculus) (not differentiated 

 in rare cases only), small intestine (duodenum, jejunum, and 

 ileum), and large intestine (colon and rectum). A caecum 

 is often present at the junction of the large and small intestine. 

 Between the stomach and duodenum as well as between the ileum 

 and large intestine there is, as a rule, a marked elevation of the 

 muscular coat serving as a sphincter (pyloric and ileo-colic 

 valves). There is also a sphincter muscle at the anus. 



The small intestine is always the longest section of the ali- 

 mentary tract: the ducts of the liver and pancreas open into 

 its anterior portion (duodenum). 



The course of the alimentary canal may be straight or more or 

 less coiled ; in the latter case it presents a greater absorptive 

 surface. As a general rule, it is longer in herbivorous than in 

 carnivorous animals. 



A considerable increase of surface also commonly results from 

 1 Comp. pp. 75 and 81. 



P 



