34 PRINCIPLES OF VETERINARY SCIENCE 



(4) Respiratory system, or organs of respiration. 



(5) Urogenital system, or organs of urination and reproduction. 



(6) Circulatory system, or organs of circulation. 



(7) Nervous system, or organs of innervation. 

 Descriptive terms are employed to indicate the exact position 



and direction of parts and organs of the body. These terms 

 refer to the animal in the ordinary standing position. The 

 surface directed toward the ground is termed inferior or ventral, 

 and the opposite surface is superior or dorsal. An imaginary 

 median plane divides the body into halves. A structure or sur- 

 face which is nearer than another to the median plane is internal 

 to it; and an object or surface which is further than another from 

 the median plane is external or lateral to it. The head end of the 

 body is termed anterior, cephalic, or cranial; and the tail end, 

 posterior or caudal. Proximal and distal represent distances from 

 the vertebral column. 



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