744 ARTIODACTYLS xxxi. i- 



the selenodont ('moon-tooth') condition. The effect is similar to 

 that arrived at, by very different means, in horses, and the enamel, 

 dentine, and cement, wearing at differing rates, provide a continually 

 roughened surface. The temporo-mandibular joint is flattened, allow- 

 ing rotary movements of the jaw, produced by the powerful pterygoid 

 muscles. 



The tongue is large and is an important part of the cropping and 

 grinding mechanism; it is very mobile, protrusible and pointed, and 

 the papillae covering it are often horny. Elaboration of the stomach 

 is common to all artiodactyls. In the pigs and hippopotamuses there 

 is a pocket close to the opening of the oesophagus and the whole car- 

 diac side secretes only mucus, pepsin being produced on the right side. 

 In the fully developed stomach of Ruminantia there are four chambers 

 (Fig. 491), rumen, reticulum, omasum (= psalterium or manyplies), 

 and abomasum. The first three are lined by a stratified epithelium of 

 oesophageal type, folded into muscular ridges. These are low in the 

 rumen, form a network in the reticulum, and are overlapping leaves 

 in the omasum. Food is first swallowed into the rumen, where it is 

 mixed with mucus and acted upon by a fauna of anaerobic cellulose- 

 splitting bacteria, whose enzymes break up the walls of the plant food 

 and reduce the whole to pulp. Organic acids, from acetic acid upwards, 

 are produced, absorbed into the circulation, and metabolized. There 

 is also a fauna of ciliates in the rumen, which digest cellulose and are 

 themselves later digested. 



The process of rumination depends upon an oesophageal groove 

 running from the cardia to the opening of the omasum. When the lips 

 of this are brought together food does not enter the reticulum and is 

 returned from the rumen to the mouth. After chewing, the bolus is 

 again swallowed, the groove opens, and the food passes to the reti- 

 culum and omasum. Here water is pressed out and absorbed and the 

 remainder proceeds to the abomasum, the 'true' stomach, with peptic 

 glands. This elaborate digestive mechanism has no doubt contributed 

 largely to the success of the artiodactyls, allowing them to eat their 

 food rapidly and then retire to digest it in security. The efficient 

 cellulose-splitting system also enables them to make use of hard 

 grasses and other unpromising sources of nutriment. 



The brain is moderately well developed in later artiodactyls, but 

 even here the cerebral hemispheres only partly cover the cerebellum, 

 and in the earlier forms the brain was relatively small, as it is today in 

 hippopotamuses and pigs. The olfactory organ and related parts of 

 the brain are well developed and most artiodactyls also have large 



