12 COMPARATIVE ANATOMY 



skeletal axis ; it is usually replaced by a vertebral column 

 consisting of rcrtebrce, at a later stage of development (Fig. 10). 

 All these structures are median in position, and the body is 

 thus bilaterally symmetrical. The neural tube, or cerebro-sp'uml 

 cavity, enclosed by the skull and arches of the vertebra:, contains 

 the central nervous system (brain and spinal cord] ; the visceral tube 

 (co:lome) encloses the viscera (alimentary canal, heart, urinogenital 

 organs, &c.), and its muscular walls may be strengthened by a 

 series of ribs, articulating dorsally with the vertebral column. 



Mtd 



KW 



FIG. 10. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE BODY OF AN ADULT 



VERTEBRATE. 



Ao, aorta ; Co, derm ; DH, lumen of intestine ; Ep, endodermic epithelium of 

 intestine; KW, body- wall ; Med, spinal cord; A/"*, mesentery; Msc, mus- 

 cular coat of intestine ; JV7?, neural tube ; Per, parietal layer of the peri- 

 toneum ; Per 1 , visceral layer of the peritoneum ; Stiln, connective-tissue 

 coat of intestine ; VR, visceral tube ; d', vertebra. 



Certain of the ribs may reach the mid-ventral line and come into 

 connection with a breast-bone or sternum, and thus form complete 

 rings or hoops around the visceral tube. 



The anterior end of the central nervous system (brain) and of 

 the alimentary tract enter into close relations with the outer 

 world, the former being connected with the higher sense-organs, 

 while from the latter are developed the mechanisms for taking in 

 nutriment and for respiration. 



The anterior portion of the body, or head, passes behind into 

 the trunk, either with or without the intermediation of a neck. The 

 coelome is practically restricted to the trunk, in the hinder part of 



