THE SPECIAL ANATOMY OF THE FROG 61 



with the tibialis anticus longus. It originates on the femur and straightens the 

 shank in the leaping and swimming movements. 



J. GENERAL ANATOMICAL PRINCIPLES 



Now that we have completed our study of the anatomy of the frog, 

 attention may be called to some of the general principles which underlie its 

 construction. 



1. Principle of bilateral symmetry. The parts of the frog are arranged sym- 

 metrically with reference to a median vertical plane which was named in the 

 early part of this outline the sagittal plane. These parts are either single 

 (unpaired) and lie in the sagittal plane which divides them into identical right 

 and left halves, or they are double (paired) and placed at the same level of the 

 body at equal distances from the sagittal plane. Unpaired structures are the 

 skull and vertebral column, brain and spinal cord, heart, digestive tract, post- 

 caval vein, and dorsal aorta. All the muscles, the appendicular skeleton, the 

 nerves and chief sense organs, most of the blood vessels, the lungs, kidneys, 

 reproductive organs and their ducts are paired. The digestive tract is the chief 

 unsymmetrical system in the body, but it obviously began as a median tube 

 extending from mouth to anus and only subsequently developed the lateral 

 displacements and spiral ceilings which have destroyed its symmetry. 



2. Principle of segmentation. Less readily recognizable is the fact that the 

 structure of the frog is based upon a repetition of parts along the sagittal axis. 

 The frog is thus conceived of as built up of a series of sections, or segments. These 

 segments are similar to each other, each has perfect bilateral symmetry and 

 contains a portion of each of the systems of the body. In the adult frog seg- 

 mentation is best illustrated by the spinal cord and its nerves, the vertebral 

 column, and some parts of the circulatory system (vessels to the body wall, 

 kidneys, and reproductive organs), all of which exhibit obvious repetition along 

 the axis, and to a less extent in the muscles (rectus abdominis and longissimus 

 dorsi muscles). The segmentation is much more complete in the tadpole. 



3. Principle of cephalization. Segmentation is retained most completely 

 in the posterior portion of the body, less so in anterior regions, and is almost 

 entirely lost in the head. Investigation shows, however, that the head like the 

 rest of the body originally consisted of a series of segments, and traces of this 

 segmentation still persist in the lobed condition of the brain, in the cranial nerves, 

 and muscles of the eye. But these segments for the most part fused together 

 in order to produce a structure, the head, which should be more specialized, 

 more efficient than the other parts over which it acquires dominance, just as 

 men and nations combine together for greater efficiency and achievements. 

 Correlated with this dominance of the head and anterior regions is a descent 

 of the viscera posteriorly. Cephalization is the name which is applied to the 

 development and specialization of the head at the expense of the rest of 

 the body. 



