274 ESSENTIALS OF ZOOLOGY 



another thin layer, the internal oblique muscle, the fibres of 

 which run in the opposite direction. The longitudinal fibres 

 of the rectus abdominis will also be seen near the medial ventral 

 region of the wall. Continuing the incision, the glistening 

 outer wall of the peritoneum will be seen. A median incision 

 through this along the linea alba, and cuts following the line of 

 the ribs, will expose the abdominal part of the body cavity and 

 its viscera, the nature and relationships of which should be 

 investigated. 



The muscles of the thoracic region have their origin in 

 the ribs. The oblique muscles of the abdomen are changed 

 into external and internal intercostals, and from this region 

 spring important muscles, of which the latissimus dorsi and 

 the pectoralis are at once prominent. But it will be better 

 now to describe the organic systems in due order. 



Alimentary Canal. The formula of the teeth of the rabbit, 



2033 



or dental formula, is *- - = 28. It means that on each 

 1023 



side of the medial line there are two incisors above, one below ; 

 no canines ; three premolars above, two below ; three molars 

 above, three below. It also indicates that the milk teeth 

 of the mammal consist of the first three groups only, viz. the 

 incisors, canines (when such are present), and premolars ; the 

 molars are not replaced. The number of the teeth and their 

 arrangement and pattern are of great importance in the classi- 

 fication and the determination of species of mammals. The 

 incisors of the rabbit, as has already been noted, grow from a 

 permanent pulp. The enamel is thick in front, and as the teeth 

 grow and are rubbed against their neighbours of the opposite 

 jaw they are maintained in a sharp chisel-like state. 



The grass or other food is nibbled off by the incisors and 

 passed to the molars to be pounded into a pulp. At the same 

 time it is impregnated with the fluid issuing into the mouth 

 from the salivary glands. 



There are four pairs of salivary glands. The parotid gland 

 lies just below the external ear, behind the mandible. Its 

 duct, Stenson's duct, runs over the masseter or cheek muscle, 

 and it opens into the mouth opposite the second premolar 

 tooth. Near it is the opening of the duct of the infraorbital 



