THE EVIDENCE OF THE THYROID GLAND 215 
secretion into the blood and lymph, and this latter secretion may be 
of the most vital importance. Now, the striking fact forces itself 
prominently forward, that the thyroid gland of the higher vertebrates 
is the most conspicuous example of the importance of such internal 
secretion. Here, although ductless, we have a gland which cannot 
be removed without fatal consequences. Here, in the importance of 
its internal secretion, we have a reason for the continued existence 
of this organ; an organ which remains much the same throughout 
the Vertebrata down to and including Petromyzon, but, as is seen 
at transformation, is all that remains of the more elaborate, more 
extensive organ of Ammoccetes. Surely we may argue that it is 
this second function which has led to the persistence of the thyroid, 
and that its original form, without its original function, is seen in 
Ammoceetes, because that is a larval form, and not a fully-developed 
animal, As soon as the generative organs of Petromyzon are developed 
at transformation, all trace of its connection with a genital duct 
vanishes, and presumably its internal secretory function alone remains. 
Yet, strange to say, a mysterious connection continues to exist 
between the thyroid gland and the generative organs, even up to 
the highest vertebrate. That the thyroid gland, situated as it is 
in the neck, should have any sympathy with sexual functions if it 
was originally a gland concerned with digestion is, to say the least 
of it, extremely unlikely, but, on the contrary, likely enough if it 
originated from a glandular organ in connection with the sexual 
organs of the paleostracan ancestor of the vertebrate. 
Freund has shown, and shown conclusively, that there is an 
intimate connection between the condition of the thyroid gland and 
the state of the sexual organs, not only in human beings, but also 
in numerous animals, such as dogs, sheep, goats, pigs, and deer. He 
points out that the swelling of the gland, which occurs in consequence 
of sexual excitement (a fact mentioned both in folk-lore tales and in 
poetical literature), and also the swelling at the time of puberty, may 
both lead to a true goitrous enlargement; that most of the permanent 
goitres commence during a menstrual period; that during pregnancy 
swelling of the thyroid is almost universal, and may become s0 ex- 
treme as to threaten suffocation, or even cause death ; that the period 
of puberty and the climacteric period are the two maximal periods 
for the onset of goitre, and that exophthalmic goitre especially is 
associated with a special disease connected with the uterus. 
