
1891.] Quantity and Dynamics of Animal Tissues. 975 
highly organized animals the differentiation of parts is often 
carried to such extremes, and the histological and anatomical 
lines of division are so ill defined, that we cannot say precisely 
where one tissue begins and another ends, and even the distinc- 
tions of a tissue as the material endowed with specific physiolog- 
ical functions is necessarily arbitrary and includes heterogeneous 
elements. To illustrate, it is not uncommon to find tables of the 
comparative weight of vertebrate brains. The value of such 
tables from the standpoint of strict homology is not very great. 
How utterly impossible it would be to find strictly homologous 
parts in all the cerebral tissues even in the different members of 
the class Mammalia! Certainly the range of physiological and 
psychological functions which these tissues discharge in the 
different members of the class is a very wide one. Some of them 
are unique and peculiar to one order, and are not represented in 
the others. Considerations like these bring us to realize that all 
quantitative studies of the tissues must necessarily proceed upon 
arbitrary assumption as to what parts are proper subjects of com- 
parison. Probably, after all, the best method of pursuing such 
studies is to compare tissues with approximately the same physio- 
logical functions. This method I have pursued in the case of a 
few of the higher animals. The weight of the skins, skeleton, 
muscles, bones, and viscera were made the subjects of comparison. 
Under the last division were included not only the viscera proper, 
but also the lungs, brain, and all parts not properly falling under 
the other divisions. Under the head of skeletal muscles, tendons 
and ligaments were necessarily included ; and under the head of 
the bones, cartilages and fatty marrows were included. Only a 
few dissections were made, and hence all inferences based upon 
them are necessarily incomplete, and require to be stated with 
extreme caution. The following comparative table gives the 
results : 3 
Animals. _. Skins, Bodies. Ratios of Skin to Body 
(approximate). 
Woodchuck, I2 oz. 80 oz. 227 
‘Rabbit, 3 oz. 23 02. 1:8 
Gray squirrel 5 oz 25 OZ. 1:5 
Black squirrel, 4 oz. 24 02. 1:6 
Skunk, © 13 OZ. 3 Ibs. 10 oz. 1:4% 


