330 DISEASE IN WILD MAMMALS AND BIRDS 
delicacy of the septa and the relative paucity of vessels is 
perhaps the reason that the gross and microscopic pic- 
tures differ from those found in mammals. It should be 
emphasized, however, that while one can perceive a hyper- 
plasia of the gland of both classes when the testes or 
ovaries are active, there appears less participation of the 
thyroid in birds in infectious diseases than is the case for 
mammals. In simple functional hyperplasia the capsu- 
lar vessels are prominent, but the cross section need show 
no change. In the continued hyperplasias the organ 
remains more solid, being less apt to develop cysts ; large 
cysts are occasionally seen, however, and in one case the 
entire gland was composed of them. Microscopically the 
differences are largely of degree in that the process is less 
frank in development, but the essential changes of swollen 
epithelia and condensed colloid remain the same. 
Atrophies. 
The thyroid gland in its functional capacity, may be 
considered to undergo hyperplasia and then atrophy of 
the parenchyma cells. Normally this would leave the col- 
loid, the epithelia and the supporting tissue in proper 
balance, but in the presence of low grade inflammation or 
where an abnormally hyperplastic process retrogresses, 
the connective tissues may exceed their norm, the 
epithelia may be shed or remain high and the colloid be 
irregular in distribution. Such a state of atrophy may 
exist in fetal life, arise from unknown cause during a 
course of toxic goitre, or perhaps insidiously in chronic 
toxic conditions. When this occurs in fetal life cretinism 
or myxedema arises, when in later life, only the latter 
appears. Judged entirely by microscopic findings, eight 
instances of atrophic changes in the thyroid have been 
found. Three of these were in Carnivora and were 
secondary to definite goitres, but were not followed by 
myxedema ; one of these three was a cretin. A brother 
of this cretin but not himself a cretin, died at the age of 
