DIGESTIVE APPARATUS. 793 



forming a valvular fibro- cartilaginous lid, that accurately closes the 

 opening of the larynx during the transit of 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. 



(2302.) The bag of the pharynx in all the Mammalia is similar in its 

 structure to that of Man ; and its muscles, namely, the stylo-pliaryn- 

 geus and the three constrictors, although stronger than in our own 

 species, offer no differences worthy of more particular notice. 



(2303.) 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 considerable 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 longitudinally, 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. 



(2304.) 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 volumes, without perhaps at 

 all advancing our real knowledge concerning the progress of digestion ; 

 we must therefore content ourselves 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. 



(2305.) 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) ; whatever its form, or the relative positions of the 

 cardiac and pyloric orifices, its structure corresponds with that of Man 

 in all essential particulars. This kind of stomach exists in by far the 

 greater number of Mammals. 



(2306.) In the complex stomach the viscus is made up of several 

 compartments communicating with each other, but without presenting 

 any difference of organization such as in the present state of physio- 

 logical knowledge would lead us to suppose them to possess different 

 functions : neither are we at all able to find any connexion between 

 such an arrangement and the nature of the substances used as food. 

 The Kangaroo (Macropus major), the Kangaroo Rat (Hypsiprymnus), 



