INTRODUCTORY 3 



as far as possible an average development in all its parts, and it must 

 not be subject to great variations in structure. 



The animal which best fulfils these conditions has been found to be 

 the Cat. The dog varies so much in size and structure that uniformity 

 is difficult to secure ; the rat is too small, and shares with the rabbit a 

 disadvantage in belonging to the rodents, a specialized group of mam- 

 mals ; the horse, cow, and sheep are inconveniently large, and are also 

 adapted to special modes of life. Even the human body, in the dis- 

 section whereof countless comparative anatomists have received their 

 first training, cannot be selected as representing a typical mamma- 

 lian form because of the changes due to the assumption of an upright 

 posture, and by reason of the reduction of some structures, and of the 

 increased complexity of the brain and many of the bones of the skull. 



GENERAL STRUCTURE OP THE CAT. 



The cat is a vertebrate ; it possesses the leading vertebrate char- 

 acter, namely, an internal skeleton, whereof the axis is the skull and 

 the jointed vertebral column, enclosing respectively the brain and the 

 spinal cord. It is, moreover, a mammal ; the females suckle their 

 young by means of mammary glands. 



Its body is composed of organs grouped into systems according to 

 the history of their development and according to the nature of the 

 work to be performed. These Systems are : 



1. The Skeletal, comprising organs of support and passive organs 

 of motion ; 



2. The Muscular, comprising the active organs of motion ; 



3. The Alimentary, comprising organs of digestion, for the recep- 

 tion and adaptation of food, and the closely related organs of respira- 

 tion, for the supply of oxygen and the excretion of carbon dioxide ; 



4. The Uro-Genital, composed of organs of excretion, for the elim- 

 ination of waste nitrogenous material, and of organs of reproduction, 

 for the propagation of the species ; 



5. The Vascular, embracing organs of circulation, for supplying to 

 the tissues, by means of the blood, oxygen and food, and for removing 

 to excretory organs the waste products ; 



6. The Nervous, comprising organs for receiving impressions of 

 different kinds ; for producing and carrying nervous impulses ; and 

 for regulating the activity of organs of the other systems ; 



