GENERAL STRUCTURE OF THE ANIMAL BODY. 3 



organs, the physical properties and structure of the tissues, as well as the 

 structure of their component elements, the cells and fibers, constitutes a de- 

 partment of anatomic science known as HISTOLOGY, or as it is prosecuted 

 largely with the microscope, MICROSCOPIC ANATOMY. 



Human anatomy is that department of anatomic science which has for its 

 object the investigation of the construction of the human body. 



GENERAL STRUCTURE OF THE ANIMAL BODY. 



The body of every animal, from fish to man, may be divided into 



1. An axial and 



2. An appendicular portion. The axial portion consists of the head, neck, 

 and trunk; the appendicular portion consists of the anterior and posterior 

 limbs or extremities. 



The axial portion of all mammals, to which class man zoologically belongs, 

 as well as of all birds, reptiles, amphibians, and osseous fish, is characterized 

 by the presence of a bony, segmented axis, which extends in a longitudinal 

 direction from before backward, and which is known as the vertebral column 

 or backbone. In virtue of the existence of this column all the class of animals 

 just mentioned form one great division of the animal kindom, the Vertebrata. 



Each segment, or vertebra, of this axis consists of 



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 v ari- 

 ously 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, however, the vertebral 

 column extends for a considerable distance beyond the trunk into the tail. 

 The vertebral column may be regarded as the foundation 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 in the head. Its walls are not only in the 

 trunk, but also 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. (See Fig. i.) Though for the most part it is quite narrow, 



