ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



members of tlu- animal kingdom. The phylum is of particular human 



interest and of special importance to the student of medicine,, for it 



includes Man, and the- study of its more archaic members affords many 



the evolutionary history of the organs met with in the human 



Tin- Yertebrata arc on the whole of relatively great size as compared 



with the invertebrates, and as they are of active habits this largeness of 



. -mparatively great complexity of structure. Amongst this 



complexity certain features stand out as specially characteristic of the 



typical vertebrate. 



The central nervous system is, as in the annelid or arthropod, con- 

 \tted into a longitudinal nerve-cord but this is characteristically 

 tubular, being perforated longitudinally by a central canal. A striking 

 physiological difference between the vertebrate and the annelid or arthro- 

 pod, and one which has had a profound influence on the evolution of 

 the phylum, is that the side of the body along which the nerve-cord runs 

 the neural side is uppermost in the normal position of the body 

 instead of being underneath, next the ground, as it is in a typical annelid 

 or arthropod. Consequently all these features of structure and they 

 , cry many which are adaptations to the ordinary position of the 

 body, are in a sense reversed as compared with the condition in annelids 

 ithropods. When the terms dorsal and ventral are used in regard 

 to a vertebrate they refer simply to the position of the body normal to 

 this phylum. The neural surface of a vertebrate is dorsal, that of an 

 annelid is ventral. 



The large size of the vertebrate and its activity of movement involve 



as in the arthropod the presence of a well-developed skeleton and the 



division of this into movable pieces, but whereas in the arthropod the 



skeleton is an exoskeleton, developed completely external to the living 



an. e, in the vertebrate on the other hand it is an internal skeleton. 



This is built up of three elements (i) the notoehord a longitudinal elastic 



rod of cells split off from the neural (dorsal) surface of the alimentary 



I ; (2) blocks of the modified connective tissue known as gristle or 



cartilage in which the rounded cells are embedded in a stiff translucent 



:i\ possessing the chemical peculiarity that it gives rise to gelatine 



when acted on by boiling water under pressure; and (3) bone also a 



lified connective tissue in which the cells are branched and the matrix 



ly infiltrated with salts of calcium. 



:miM ular system of the vertebrate consists for the most part 

 Of development of longitudinal fibres arranged 

 le of the body in segmental blocks or myotomes. 



