MAMMALIA. 677 



food into the throat. The communication between the posterior 

 nares and the faucial cavity is likewise protected by a musculo- 

 membranous valve, called the velum pendulum palati ; but as, 

 with the exception of the CETACEA hereafter to be noticed, the 

 arrangement of these parts exactly resembles what is seen in the 

 human subject, it would be superfluous to describe them more 

 minutely in this place. 



(758.) The bag of the pharynx in all the Mammalia is similar 

 in its structure to that of Man ; and its muscles, namely, the 

 stylo-pharyngeus, and the three constrictors, although stronger 

 than in our own species, offer no differences worthy of more parti- 

 cular notice. 



(759.) The oesophagus, leading from the termination of the 

 pharynx into the stomach, is a long muscular tube, that traverses 

 the chest in front of the bodies of the dorsal vertebrae, and, having 

 pierced the diaphragm, reaches the abdominal cavity. Its lining 

 membrane is loose and much plicated, so as to allow of consider- 

 able dilatation ; but externally its walls are very muscular, the 

 surrounding muscles being arranged in two distinct layers. In 

 Man the outer stratum of muscular fibres is disposed longitudi- 

 nally, while the inner layer consists of circular fibres ; but in 

 most other Mammalia both these layers assume a spiral course, 

 and cross each other obliquely as they embrace the cesophageal tube. 



(760.) The stomach itself presents such endless diversity of 

 form, that merely to enumerate all the details that have been 

 amassed relative to this part of our subject would fill many vo- 

 lumes, without perhaps at all advancing our real knowledge con- 

 cerning the process of digestion ; we must, therefore, content our- 

 selves with a very general view of the organization of this important 

 viscus, and regard the Mammalia as possessing either simple, 

 complex, or compound stomachs, each of which will deserve a 

 distinct notice. 



(761.) In the simple form of stomach the organ consists of a 

 single cavity, as is the case in the human species, let the shape 

 of the viscus be elongated, pyriform, or globular; for in this 

 respect there is every possible variety ; but whatever its form, or 

 the relative positions of the cardiac and pyloric orifices, its struc- 

 ture corresponds with that of Man in all essential particulars. This 

 kind of stomach exists in by far the greater number of Mammals. 



(762.) In the complex stomach the viscus is made up of several 

 compartments communicating with each other, but without pre- 



