THE GROUND PLAN OF ORGAN SYSTEMS. 43 
On the other hand in ordinary description of organs and their 
position, where it is not a matter of the official names of parts, little 
advantage is to be gained from adherence to this principle. The 
terms anterior and posterior apply with much greater force to a 
lower vertebrate, than to man, while the terms superior and 
inferior are only of interest in the latter. In this case the rule here 
followed is to use the terms anterior and posterior for descriptive 
purposes without reference to the human relation. The same 
remark applies to the terms of direction, viz., upward, downward, 
forward, and backward. 
In this connection it may be pointed out that the custom has 
become more or less general in comparative anatomy of employ- 
ing the termination ad with words otherwise signifying position 
alone, in order to indicate position or course toward, e.g., dorsad = 
dorsalward. In the present case this form is used only for course, 
position being indicated by the adverbial termination ly, e.g., 
dorsally. 
Reference may also be made here to the fact that the human 
structures to which identifying names are applied frequently fail in 
one way or another to correspond to structures in a lower form. 
Composite structures to which individualizing names are applied, 
for example, may be represented by independent parts. Also, 
structures which are similar in form or function may be convergent. 
Finally, although it is essential to endeavour to apply all terms as 
accurately as possible, it will be remembered that a terminology 
primarily arranged for one type cannot be exactly applied to 
another without considerable qualification. 
THE GENERAL FEATURES AND GROUND PLAN OF 
THE ORGAN SYSTEMS. 
It has already been stated as a general principle that the 
structure of an organism is the expression of an underlying plan and 
pattern, in the elaboration of which embryonic development and 
ancestry play a very large part. How, asa matter of interpretation, 
the comparative method is applied, may be demonstrated by 
reference to any part of the body of an animal; and in the following 
pages will be found, under the head of the respective systems, a 
