8 Anatomy of the Rabbit. 



Many of the more general features of the rabbit are such as are 

 not recognized by group designation, but yet are shared with other 

 terrestrial vertebrates, including mammals, reptiles, birds, and, in part, 

 amphibians. This refers to the development of the lungs and associated 

 respiratory tracts, both the true respiratory tracts and the accessory 

 respiratory passages traversing the skull ; further, the loss of the branchial 

 or fish-type of respiration and the new disposition of the branchial 

 structures; the development of a tri-segmented type of limb with a full 

 complement of muscles, and originally a pentadactyl, plantigrade foot, 

 for support of the body and for locomotion. The regional differentiation 

 of the vertebral column, especially the mobility of the neck, the free 

 occipital articulation, and the definition of the sacrum, the latter associa- 

 ted with the elaboration of the pelvic girdle, are all features of general 

 significance in the terrestrial vertebrates. 



Finally, the rabbit agrees with other members of the phylum Chordata 

 in the possession of a ground-plan underlying the most general features of 

 its organs, and the position, arrangement, and plan of development of its 

 organ-systems. All Vertebrata or back-boned animals possess an axial 

 skeleton formed by the segmented vertebral column. In a very com- 

 prehensive way they possess as chordates a still more fundamental 

 axial support, the notochord, the latter being an embryonic structure 

 except in the lowest chordates. In a more restricted sense, as Craniota, 

 they possess an organized head region with differentiated brain, 

 special sense organs, and enclosing primary skull. They possess a series 

 of branchial (branchiomeric) structures,"appearing either in the adult 

 condition, as in fishes, or as part of the underlying plan in the embryonic 

 condition; and they add to their general features in the arrangement of 

 the organ-systems the further feature of transverse segmentation 

 (metamerism) of a considerable portion of the body. 



GENERAL ANATOMY. 



Although in every respect a continuous structure, the body is differ- 

 entiated into a large number of parts, or organs, the latter being 

 more or less individual in form, composition, or function. Organs 

 are arranged for the most part in systems, each of which is concerned 

 with some general or fundamental function, to which several organs 

 may contribute. 



In a more general way the body may be considered as an assemblage 

 of tissues. The latter are layers or aggregations of similarly differen- 

 tiated \ cells. They are of several different kinds, and are variously 

 associated in the formation of organs. Being structures of an inter- 

 mediate position they may be considered either as organ components 

 or as products of specialized cells. 



As a body-unit a cell consists of a small mass of living protoplasm, 

 containing a central body, the nucleus, and surrounded or enclosed on 

 its free border by a cell -membrane. The nucleus is a highly organized 

 body, having an important function in the reproduction of the cell and 



