146 SECTIONAL ADDRESSES 
formation around blood vessels is a striking feature. In the mammalian 
pituitary the circulation is sinusoidal, and provision is made for the 
cellular secretion to enter directly into the blood. ‘The epithelial lobe 
is obviously an endocrine gland which pours its products into the general 
circulation. Its secretion would, therefore, appear to be destined more 
for general purposes in the body than for action localised to its immediate 
neighbourhood. 
The pars nervosa, when suitably fixed and stained, holds masses of fine 
granular particles in especial abundance around the blood vessels. These 
granules probably represent a storage of the secretion peculiar to the lobe. 
Their mode of origin is not yet settled. ‘They may be derived from the 
epithelial investment, and more particularly from the breaking down of 
the epithelial cells which penetrate the pars nervosa from the pars inter- 
media, but the granules are found in all parts of the lobe, and not merely 
in the neck where penetration by epithelium is most evident. A study 
of the comparative anatomy of the posterior lobe, especially in reptiles 
and birds, suggests that the nerve elements play some part, and that the 
modified neuroglia or ependyma cells may be secretory. There are 
examples of secretion by cells of nervous origin, the chromophil cell of 
the medulla of the suprarenals being a prominent one. ‘The pineal body 
contains material which acts upon the melanophores of the tadpole, and 
there appears to be no cell other than the parenchymatous, a modified 
form of neuroglia, to account for it. 
The hormones of the posterior lobe differ in certain respects from those 
of the pars intermedia. The pressor substance is peculiar to the pars 
nervosa, occurring in the pituitary body of all vertebrates which possess 
a pars nervosa, and absent from those which have none. On the other 
hand many of the activities possessed by extracts of the posterior lobe are 
also shown by extracts of the pars intermedia, though as a rule less strongly. 
It is probable that this material is elaborated by cells of the pars inter- 
media, pars tuberalis, and even by cells of the pars glandularis. Hyaline 
material is of common occurrence, especially in the neck of the posterior 
lobe, and similar material is found in acinar-like structures in the pars 
tuberalis and pars intermedia. In some animals, and occasionally in the 
human pituitary, the formation resembles small thyroid vesicles. The 
material is sometimes a secretion and at others a product of the actual 
breaking down of cells which have wandered into the pars nervosa. In 
some cases it has been traced into the veins of the lobe, and from there 
to the adjacent wall of the brain. Its origin is clearly epithelial. 
Three ways are open to the secretion of the posterior lobe: direct 
absorption into the blood vessels ; penetration into the adjacent nervous 
floor of the tuber cinereum; and direct entry into the cerebrospinal 
fluid. There is evidence that all three routes may be taken. The histo- 
logical features of the posterior lobe are suggestive of its being a brain 
gland which furnishes a secretion for local action in the diencephalon. 
The blood vessels of the lobe are relatively much less numerous than those 
of the anterior lobe, and are capillaries, not sinusoids. ‘Their arrangement 
is peculiar. Popa and Fielding have described a venous portal system in 
the human pituitary body. Capillaries in the pars nervosa, intermedia 
