Anatomical and physiological studies of the vascular, nervous and muscular tissues of the mammary glands

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