PRINCIPLES OF VETERINARY SCIENCE 



PART I 



ANATOMY AND PHYSIOLOGY OF ANIMALS 



CHAPTER I 



CHARACTERISTICS OF ANIMALS 

 THE PLACE OF ANIMALS IN NATURE 



The animal kingdom includes all animal life. It is divided 

 into two subkingdoms: (1) vertebrates, (2) invertebrates. 

 Veterinary science is principally concerned with the former. 



The vertebrates are distinguished by having a vertebral col- 

 umn or backbone composed of segments known as vertebra. 

 They are further characterized by never having more than two 

 pairs of limbs, which are always attached to the lower part of 

 the body. 



In fetal life the body of the vertebrate is traversed by an un- 

 jointed cartilaginous rod, the notocord. In the lower vertebrates 

 this rod persists throughout life, but in the higher it is replaced 

 by the true vertebrae (Fig. 1). 



A cross-section of the body of a vertebrate animal shows two 

 cavities, with the vertebral column lying in the partition between 

 the two. The upper cavity is tube-like in form and contains 

 the spinal cord and brain of the central nervous system. The 

 lower cavity contains the alimentary canal, heart, lungs, and 

 other soft organs. Invertebrates have but one cavity which 

 encloses both the nervous system and the soft organs. 



Vertebrates are divided into the following five classes: (1) 

 fishes; (2) amphibians; (3) reptiles; (4) birds; (5) mammals. 

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