138 DONALDSON. [Vor. IX. 
Age.—In our experiments the age of the animals, which 
were obtained from the slaughter-house, was inferred from 
the weight of the brain. They were mainly wethers from 
the Cotswold breed of sheep. 
A series of 52 sheep’s brains gives the following statistics : 
The heaviest hemiencephalon was 69.65 grms., and the lightest, 
43.17 grms. The left hemiencephalon in the case of 40 brains 
averaged 56.4 grms., and the right hemiencephalon 56.1 grms. 
The so-called lambs’ brains were 12% lighter. 
The attempt was repeatedly made to’ determine whether the 
fresh weight of the sheep’s brain or the size of the bosses on 
the skull, both considered as indices of age, bore any relation 
to the amount of the increase in weight in the bichromate 
solutions, but no such relation could be found. 
Neither was it found that the percentage of solids was 
correlated with the fresh weight of the brain. 
We conclude, therefore, that the differences in brain weight 
due to the ages within which our specimens ranged, are not 
significant for our purpose, nor is the difference above noted 
in the weights of the two halves of the encephalon indicative 
of anything, save the difficulty of dividing the brain evenly. 
Season. — The brains were placed in the hardening solutions 
at different times of the year without finding any variations to 
be associated with the seasons. 
Influence of Pressure. — Of the minor conditions which influ- 
ence the change in weight, pressure, as we have just pointed 
out, in speaking of specimens with the pia intact and those 
with it slit, is the most important. If a specimen be pressed 
against the containing vessel, or bound about in any way, two 
things happen, — it swells less than under ordinary conditions 
and hardens more slowly or not at all, according to the degree 
to which it is compressed. 
The specimens were all treated without removing the pia. 
The pia mater is but a slightly extensible membrane, conse- 
quently when an encephalon with the pia intact is put into a 
solution of bichromate of potash, it does not increase in weight 
so much or harden so quickly as another encephalon in which 
the pia has been slit. 
