XXVlll INTRODUCTION. 



nium encloses the brain, the substance of which is prolonged 

 through the canal of the vertebral column, forming the spinal 

 chord, terminating in the lower vertebrae. From the under 

 part of the brain, and from the spinal chord, proceed bundles 

 of nervous filaments, which, dividing, subdividing, and inter- 

 mixing, communicate with every sensible part of the body. 



All the vertebrata have ribs, or hoops of bone, for protect- 

 ing the lungs and other organs, with the exception of a few 

 tribes in which the ribs are rudimental. Their limbs con- 

 sist of two pairs, though one and sometimes both pairs are 

 rudimental or wanting. The upper or anterior limbs may 

 be arms and hands, as in man and the monkey tribes ; legs 

 and feet, as in quadrupeds ; organs for flight, as in birds ; 

 fins, as in fishes : the hinder or inferior limbs may be feet, 

 legs, or fins, according to the uses to which they are des- 

 tined. 



All the vertebrata have a muscular organ, the heart, con- 

 tained within the chest, for propelling the blood through the 

 system. They have all respiratory organs, in which the 

 blood, passed through innumerable capillary tubes, finer 

 than the finest hair, is acted upon by the air of the surround- 

 ing medium. In fishes, and certain reptiles, the respiratory 

 apparatus is termed branchite or gills, over which the water, 

 containing air, passes ; in all the other vertebrata, the res- 

 piratory apparatus, termed lungs, consists essentially of a 

 congeries of minute cells, into which the air is drawn through 

 the trachea or windpipe from the mouth and nostrils. 



In all the vertebrata there is a continued canal, which, 

 commencing at the mouth, extends through the body, and 

 which, enlarging within the abdomen, forms the stomach, 

 consisting of one or more cavities, in which the food is re- 

 tained for a time. The food is then acted upon by various 

 fluids, secreted from the interior surface of the stomach, by 

 the action of which it becomes a pulpy mass, to which is ap- 

 plied the term Chyme. The chyme thus formed passes on- 

 wards by the extremity of the stomach towards the remain- 



