42 Anatomy of the; Rabbit. 



of the ascending limb of the duodenal loop. The course of the caecum as 

 it lies in the body is comparable to two turns of a left-hand spiral, its blind 

 termination, the vermiform process, being dorsal in position and directed 

 for the most part backward. It may be observed at this point that in 

 their vascular supply the more typical divisions, namely, the transverse 

 and descending colons, have arterial branches, respectively, the middle and 

 left colic arteries, comparable to those of man ; while on the other hand 

 the right colic relation, on account of the great complexity of its parts, is 

 represented by a large number of vessels, branches of a common ileo- 

 caecocolic trunk. 



The form of the anterior, or cephalic portion of the digestive tube 

 (Plate II) depends on its fixed relation with respect to the enclosing 

 parts of the head-skeleton. In the rabbit, as in mammals generally, the 

 oral cavity is divisible into two portions, of which one is the oral cavity 

 proper, while the other, the vestibulum oris, is a space enclosed betw^een the 

 alveolar process of the jaws and the teeth on the one hand and the cheeks 

 and lips on the other. As in other vertebrates, the tongue is a muscular 

 structure projecting upward and forward into the oral cavity from its 

 base of attachment on the hyoid apparatus, but its greater elaboration, 

 as well as the differentiation of special processes, the circumvallate 

 and foliate papillae, for the accommodation of the gustatory organs, are 

 features of mammalian significance. The roof of the oral cavity is 

 formed by an extensive palatal surface, comprising the hard palate, 

 and the membranous, or soft palate. These structures also form the floor 

 of the accessory respiratory tracts of the nose, the posterior aperture 

 being thus carried backward to a point more directly above the aper- 

 ture of the larynx. 



As accessory structures the teeth present two mammalian features; 

 they are heterodont, or differentiated according to particular regions, and 

 the adult series, excepting those designated as molars, are permanent 

 teeth, replacing deciduous, or milk teeth of the young animal. The 

 condition is thus described as diphyodont in comparison with that in 

 lower vertebrates, where there is usually a multiple tooth change, new 

 teeth being developed as required (polyphyodont type). Moreover, in the 

 rabbit, as in all mammalia, the number is restricted, so that, considering 

 the differentiation of the teeth, it is possible to express their relations 

 by a dental formula. In the mammalia generally the teeth are differen- 

 tiated into incisors, canines, premolars and molars, and in placental 

 mammals the full dental formula is indicated as i. 4}, c. }, pm. i, m. |. 

 In the rabbit as in other rodents, however, the dentition is greatly 

 modified by the elaboration of two pairs of incisors and the corresponding 

 obliteration of intermediate teeth, the place of the latter being occupied 

 by an extensive gap, or diastema in which no teeth occur. The dental 

 formula of the rabbit is i. f, c. %, pm. |, m. f . It will also be observed 

 in this animal that the absence of the intermediate teeth allows the lips to 

 be approximated behind the incisors, and since the lips are in this region 

 also provided with hairs on their internal surfaces, the oral cavity ig 

 separated almost completely from a small space enclosing the incisor 

 teeth. This adaptation, however, is not so perfectly developed in the 

 rabbit as in certain others of the rodent order. 



