INTRODUCTION 3 



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. 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 can 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 for 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. 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 con- 

 tained a musculo-membranous 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 ex- 

 tremity 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 Surfaces of the Body. The external 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 latter of layers of scales or cells. Em- 

 bedded within the skin are numbers of glands, which exude, in the different 

 classes of animals, sweat, oily matter, etc. Projecting from the surface of 



