Vlll INTRODUCTION. 



recurved hornj- spines, used to scrape the last fragments of flesh off 

 bones. 



The tongue is supported by a bone called the Hyoid bone, the 

 anterior liorns of which, small in Man, are greatly developed 

 in most Mammals. The velum palati, which protects the com- 

 munication between the mouth and posterior nasal cavity is only 

 found in this class ; and the trachea, which lies in front of the 

 oesophagus, is protected by the epiglottis. The oesophagus leads 

 straight to the stomach from the end of the pharynx, and is of 

 moderate width, but dilatable. The stomach presents great varie- 

 ties of form. In most it is a simple bag of varied shape and 

 size, in some divided into compartments bj' constrictions, but 

 without any apparent difference of structure. In one tribe, how- 

 ever, the Ruminants, it is a truly compound structure, consisting of 

 several distinct cavities differing both in size, structure and func- 

 tions. The small intestines vary little, but the large intestines are 

 very variable in size, and the division between them is in many 

 marked by an appendage called the cxoum, sometimes double, and 

 in certain animals of great size. In some, especially the Planti- 

 grade Carnivora and Cetacea, it is entirely absent. 



The liver is generally of large size, and is usually divided into 

 several lobes. In most it is furnished with a gall-bladder. The 

 pancreas and spleen are always present. 



The inferior muscular energy of Mammals compared with that of 

 13ird.s is accompanied by an inferior amount of respiration, and on 

 this account the heat of animals is much less, rarely exceeding 

 100° of Fahrenheit. The organs- of respiration and circulation 

 differ little throughout the class from those of Man. The heart 

 consists of four cavities ; and the lungs, which are always in pairs, 

 arc completely enclosed, and have no communication with air-cells 

 as in Birds. These viscera are separated from those of the abdomen 

 by the diaphragm, a muscular partition, which is one of the chief 

 agents in respiration. The lungs are more porous and spongy in 

 texture than in Birds. Variations in the structure of the heart and 

 circulating system are only met with in Cetaceous animals, which 

 have some interesting peculiarities dependent on their remaining 



