INTRODUCTION. 3 



1. A solid portion, known as the body or centrum, and 



2. A bony arch arising from the dorsal aspect and surmounted by a spine- 

 like process. 



At the anterior extremity of the body of the animal the vertebrae are 

 variously modified and expanded, and, with the addition of new elements, 

 form the skull; at the posterior extremity they rapidly diminish in size, 

 and terminate in man in a short, tail-like process. In many animals, how- 

 ever, the vertebral column extends for a considerable distance beyond the 

 trunk into the tail. The vertebral column may be regarded as the founda- 

 tion element in the plan of organization of all the higher animals and the 

 center around which the rest of the body is developed and arranged with a 

 certain degree of conformity. In all vertebrate animals the bodies of the 

 segments of the vertebral column form a partition which serves to divide 

 the trunk of the body into two cavities viz., the dorsal and the ventral. 



The dorsal cavity is found not only in the trunk, but also in the head. 

 Its walls are formed partly by the arches which arise from the posterior 

 or dorsal surface of the vertebrae and partly by the bones of the skull. If a 

 longitudinal section be made through the center of the vertebral column, 

 and including the head, the dorsal cavity will be observed running through 

 its entire extent. Though fo$ the most part it is quite narrow, at the anterior 

 extremity it is enlarged and forms the cavity of the skull. This cavity is 

 lined by a membranous canal, the neural canal, in which are contained the 

 brain and the neural or spinal cord. Through openings in the sides of the 

 dorsal cavity nerves pass out which connect the brain and spinal cord with 

 all the structures of the body. 



The ventral cavity is confined mainly to the trunk of the body. Its 

 walls are formed by muscles and skin, strengthened in most animals by bony 

 arches, the ribs. Within the ventral cavity is contained a musculo-mem- 

 branous tube or canal known as the alimentary or food canal, which begins 

 at the mouth on the ventral side of the head, and, after passing through the 

 neck and trunk, terminates at the posterior extremity of the trunk at the 

 anus. It may be divided into mouth, pharynx, esophagus, stomach, small 

 and large intestines. 



In all mammals the ventral cavity is divided by a musculo-membranous 

 partition into two smaller cavities, the thorax and abdomen. The former 

 contains the lungs, heart and its great blood-vessels, and the anterior part of 

 the alimentary canal, the gullet or esophagus; the latter contains the con- 

 tinuation of the alimentary canal that is, the stomach and intestines 

 and the glands in connection with it, the liver and pancreas. In the 

 posterior portion of the abdominal cavity are found the kidneys, ureters, 

 and bladder, and in the female the organs of reproduction. The thoracic 

 and abdominal cavities are each lined by a thin serous membrane, known, 

 respectively, as the pleural and peritoneal membranes, which, in addition, 

 are reflected over the surfaces of the organs contained within them. The 

 alimentary canal and the various cavities connected with it are lined through- 

 out by a mucous membrane. 



The surface of the body is covered by the skin. This is composed 

 of an inner portion, the derma, and an outer portion, the epidermis. The 

 former consists of connective tissue fibers, blood-vessels, nerves, etc.; the 



