18 ! Mr. Moncax's Description 
compressing muscles through which they pass ; and it therefore 
appears more than probable, that whenever the gland is squeezed 
against the marsupial bone, a greater or less degree of venous 
congestion, and consequently distention of the part, must be 
occasioned by the pressure which is made upon the veins 
through which the blood is returned. That a loaded state of 
the veins, together with an injection of the lactiferous tubes, 
. will occasion in the marsupial gland of the dead animal an in- 
crease of size corresponding to that which is found to exist jin 
the parts during life, I have proved by the experiment of throw- 
ing an injection of quicksilver into the ducts, and one of water 
into the blood-vessels, by which process the exact natural form 
and capacity of the mamma, as it exists during the period of 
suckling, is artificially produced. "Thus the extraordinary dis- 
tention of the marsupial mammary gland to which I have alluded, 
is, I conceive, produced in a great measure by an enlargement 
of the vessels which naturally exist in the part; but the extra- 
ordinary distention of the nipple is partly occasioned by a change 
which takes place in a peculiar vascular structure which enters 
into the composition of the teat, and which is formed apparently 
for this particular purpose: for we find immediately beneath the 
compressing muscle of the teat, that a layer of loose reticular 
membrane, forming a bed for a congeries of tortuous veins, is 
interposed between that structure and the central fasciculus of 
excretory ducts. The vascular sheath by which this central 
fasciculus is thus inclosed, consists principally of a dense plexus 
of veins, which are extremely large and numerous in proportion 
to the size and number of the arteries which accompany them 
(tab. 8. f. 2. c.). So great is the vascularity of this sheath, that in 
many parts it nearly resembles in appearance the corpus spon- 
giosum of the penis, and like that part is capable of considerable 
distention, either by an obstruction to its venous circulation in 
the 
