It is not necessary to reiterate in detail the gross blood
and nerve supply to the mammary glands given in Section I, Part
II. The glands extend along the ventral thorax and abdomen and
receive a multiple blood and nerve supply in common with the
other subcutaneous tissues.The fact that many cutaneous nerves of thoracic and lumbar
!origin innervate the glands, stresses the difficulty of thorough
ly denervating them for experimental purposes, without actual
transplantation.The multiple blood supply is understandable in the case of
such a widely dispersed tissue, but it may have additional significance for maintaining the blood flow under all conditions.
,For example, it has been shown that the anastomoses within the
tissue are such that it is possible for blood to reach all parts
from any one main source of supply. It is conceivable that in
,certain positions of the body some of the mammary vessels,which
are very superficial, might be sufficiently compressed to reduce
the blood flow through them. However the vascular anastomoses
mean that it is very unlikely that this would affect the total
flow through the tissue as a whole.The microscopical examination of the small sblood vessels
shows that, whilst the lobes have a multiple blood supply, the
individual lobules do not generally receive more than one arteriole and venule. The capillaries around the alveoli form part
of a complete network confined to the individual lobules but
also embracing the smallest milk ducts draining them. The
larger ducts also have an encircling capillary net, which is
supplied at intervals by arterioles and venules, and continues
right up to the mouths of the ducts, when it joins that of the
skin. The largest ducts and cisterns have in addition a second
layer of vessels, formed by the supplying arterioles and venules.The arteriole-venular bridges of zweifach (1939), seen in
some lobules and the arterio-venous anastomoses doubtfully
recorded on some of the ducts, probably serve to maintain the
overall blood flow through the tissues, whilst allowing greater
control of the flow through small parts of it.The significance of the venous network in the teat is not
clear. It was first thought by Furstenberg (1868), Riederer
(1903) and _ubeli (1916) to form a cavernous erectile tissue in
the cow and that it was concerned in the flow of milk. Purstenr
berg for example, thought that a cow held up her milk by holding
her breath and actually obliterating the teat lumen by venous
turgescence, whilst Rubeli believed that the latter was produced by vasomotor nerves. As has been shown in Section VIII,
Part I and in Section IV, Part II of this thesis, it is no
longer thought that the holding up of milk is an active process,
but that the "letting down" is. The teat vessels have received
no mention in modern theories, neither have they in the recent
studies of the erection of the teat and its behaviour during
milking by Peeters, Massart, Oyeart and Coussens (1948). They
have shown that the smooth muscle in the nipple undergoes
rhythmical contractions when it is distended, and suggested that
the compression of the veins at this time aids the return of
the blood to the heart
