422 MANUAL OF ELEMENT A R Y ZOOLOGY 



each sheet forming the inner wall of a pleural cavity.- 

 Between the sheets is a lymph-space known as the 

 mediastinum. In the dorsal part of this space He the aorta, 

 certain other blood vessels, and the oesophagus ; its middle 

 part is quite filled by the pericardium, with which it fuses ; 

 and in its ventral part lies the thymus. 



The skeleton of the rabbit in its main features, and to 

 a considerable extent in its details, resembles 



Backbone. tnat °f tne fr °g> but on 'y m * ts broadest out- 

 lines can a correspondence with that of the 

 dogfish be traced. It is almost entirely bony, though most 

 of it is first laid down in cartilage, which persists upon the 

 surfaces of the joints and elsewhere. The vertebrae x are 

 much like those of the frog (p. 27), each of them being 

 entirely bony and consisting of a body or centrum with two 

 neural arches, which enclose above the centrum a vertebral 

 foramen, surmounted by a neural spine or spinous process. 

 As in the frog, each arch bears in front an upward-facing 

 facet or superior articular process or prezygapophysis and 

 behind a downward-facing inferior articular process or post- 

 zygapophysis which fits on to the corresponding prezyga- 

 pophysis of the next vertebra, while at the side a transverse 

 process projects, and at each end there is an intervertebral 

 notch for the passage of a spinal nerve, the adjacent 

 notches of two vertebra enclosing an intervertebral foramen. 

 Each end of each centrum, with the exception of the first, 

 is flat, and has applied to it in the young rabbit a thin 

 bony disc or epiphysis, which fuses with it when growth is 

 complete. There is more difference between the vertebras 

 than in the frog, the backbone being divided into five 

 sections, the neck or cervical, chest or thoracic, loin or 

 lumbar, hip or sacral, and tail or caudal regions. In the 

 cervical region there are seven vertebras, which may be 

 recognised by the fact that each of the transverse processes 

 is pierced by an opening, known as its foramen, through 

 which the vertebral artery passes, so that there is formed 

 an interrupted vertebrarterial canal on each side. This 

 is due to the fusion with the vertebras of short cervical 

 ribs in such a way as to constitute a compound transverse 



1 The general characters of the vertebra of the rabbit may be Well 

 studied in that known as the second lumbar (see below). l 



