1306 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



The nerves. — The gland proper receives its nerves laterally from the lateral mammary rami 

 of the anterior rami of the lateral cutaneous branches of the fourth to sixth intercostal nerves 

 and medially from the medial mammary rami of the anterior cutaneous branches of the second 

 to the fourth intercostal nerves. The skin over the breast receives in addition to branches from 

 the above nerves, branches from the supraclavicular nerves of the cervical plexus. It is alto- 

 gether probable that sympathetic fibres reach the gland but by what course is not yet clear. 

 The nerves are distributed in part to the skin, in part to the plain muscle of the areola and nip- 

 ple, some to the blood-vessels, and others to the glandular tissue. The secretion is, however, 

 not entirely controlled by nerves as it is influenced also by hormones from other organs brought 

 to it by the blood. 



Development. — In very early embryos the epithelium over an area on the side of the body 

 extending from the fore to the hind limb (or bej^ond these limits) is seen to be deeper and more 

 cubical, the mammary streak. In this area there is produced by multiplication of cells a ridge, 

 the mammry line or ridge. In spots along this line, corresponding to the relative position of 

 the mammary glands in some mammals and the supernumerary mammae in man, the epithelium 

 thickens. The intervening parts of the line disappear as the spots enlarge to form transient 

 mammary hillocks. In man ordinarily development proceeds in but one of these hillocks on 

 each side. The deep surface of the hillock projects into the corium as the superficial surface 

 flattens out and the mesodermic cells of the corium condense around the ingrowth producing 

 the nipple zone. Rapid proliferation of the deeper cells produces a club-shaped stage from the 

 deeper surface of which small«bud-like masses of epithelial cells sprout and extend as solid plugs 

 into the corium. These are the anlages of the true secreting part of the gland and the number of 

 buds corresponds to the number of lobes of the future gland. The sprouts extend beyond and 

 beneath the nipple zone and are supported by closely packed connective-tissue cells forming the 

 stroma zone. The epithelial buds continue to grow and branch and a lumen is finally produced 

 in the originallj'- solid plugs. The primary epithelial ingrowth degenerates and ultimately dis- 

 appears. A cavity is produced in it which later connects with the lumina of the gland ducts. 

 The depressed nipple zone becomes elevated above the surface soon after birth. Further 

 development of the mammary gland has been discussed previously under changes due to age 

 and functional activity (p. 1303). 



THE DUCTLESS GLANDS 



Under the term ductless glands are included not only certain glandular struc- 

 tures of epithelial origin with a more or less definitely known function and an 

 internal secretion but also certain organs whose function is not definitely known or 

 understood. Of the organs here considered, the function of the thyreoid gland, 

 the parathyreoid glands, the chromaffin system, the medullary portion of the 

 suprarenal glands, and the aortic paraganglia is somewhat definitely known. 

 But the function of the thymus, the spleen, the cortical portion of the superenal 

 glands, the glomus caroticum, and the glomus coccygeum is still in doubt; 

 although probably some, if not all of them, have an internal secretion or at any 

 rate are closely associated with the other glands of internal secretion. The hypo- 

 physis and the pineal body are not considered in this connection but will be found 

 described with the brain (pp. 845,848). The lymph-nodes, which may also 

 be considered as ductless glands, are described in Section VI. Many of the 

 true glands, such as the liver, pancreas and sexual glands, have also internal secre- 

 tions which pass directly into the vascular system as in the ductless glands. 



THE SPLEEN 



The spleen [lien] is a large blood-vascular organ closely associated with the 

 lymphatic sj'stem. Its exact function is still in doubt. 



Position. — The spleen is situated in the dorsal part of the left cephalic segment 

 of the abdominal cavity so deeply placed against the diaphragm and dorsal to the 

 stomach and colon as to be invisible from the ventral surface of the body when the 

 abdominal cavity is opened. It is mainly in the left hypochondriac region but its 

 deepest and most cephalic part extends also into the epigastric region. It is 

 obliquely placed with its long axis corresponding approximately to the line of the 

 caudal ribs. It tends to become more vertical when the stomach is fully distended 

 but when the stomach is em])ty and the colon distended it assumes a more hori- 

 zontal position. Changes in the attitude of the body also cause slight altera- 

 tions in the situation of the spleen. It moves with the excursions of the dia- 

 phragm in expiration and inspiration. 



The colour of the spleen is, in life, a dark bluish-red or brownish-red, but after 

 death it becomes darker with a more bluish or violet tint. 



The size of the spleen is perhaps more variable than that of any other large 



