THE UPPER LIMB 303 



Development of the Mammary Glands. — The mammary glands are of ecto- 

 dermic origin. In the course of the second month of intra-uterine life a 

 circular thickening of the epidermis (ectoderm) makes its appearance in the 

 region of the future gland. This thickening projects into the subjacent 

 mesodermic corium, and the superficial cent? al cells of the thickening becom- 

 ing shed, the thickened area is now depressed. From the deep surface of the 

 thickened ectodermic area solid columns of cells grow into the subjacent 

 mesodermic corium, which give oflE lateral offshoots. These cell columns 

 become hollow, and form the lactiferous or galactophorous ducts, which open 

 upon the area corresponding to the circular thickening of epidermis. The 

 lateral offshoots of the cell columns give rise to the lobes, lobules, and alveoli 

 or acini of the gland. Indeed, each lateral offshoot gives rise to a mammary 

 gland in miniature. 



The nipple and areola are developed from the glandular area — that is to 

 say, the area of the original thickening of epidermis. The depression of this 

 area disappears, and its central portion becomes elevated to form the nipple. 

 This consists of epidermis, and contains the terminal parts of the lactiferous 

 ducts, bloodvessels, and connective tissue. The remainder of the glandular 

 area forms the areola. The corium corresponding to the nipple and areola 

 is provided with unstriped muscular itssue. 



The connective-tissue stroma of the gland is developed from the mesoderm 

 of the corium. 



Pectoralis Major — Origin. — (i) The anterior surface of the 

 clavicle over its inner half, and the anterior stemo-clavicular 

 ligament; (2) one-half of the anterior surface of the sternum as 

 low as the metastemum; (3) the anterior surfaces of the upper 

 six costal cartilages, and slightly from the bony part of the sixth 

 rib; and (4) the upper portion of the external oblique aponeurosis. 



Insertion. — The outer bicipital ridge of the humerus over its 

 lower three-fourths. 



The muscle is divisible into two portions, clavicular and sterno- 

 costal, which are separated by a slight interval extending down- 

 wards and outwards from the stemo-clavicular joint. 



The tendon of insertion is folded upon itself so as to be composed 



3f two layers, anterior and posterior, the former of which is the 



shorter, and both being continuous below. The anterior layer 



receives the stemo-costal fibres above the third costal cartilage, 



md it also receives superficially the claviciilar fibres, which latter 



lescend lowest at their insertion, where they are intimately con- 



lected with the tendon of the deltoid. The posterior layer receives 



ill the fibres from the third costal cartilage downwards, and the 



owest of these fibres, as they pass outwards and upwards, become 



uccessively folded underneath the fibres above. The result is 



hat the fibres arising lowest reach the highest part of the posterior 



:iyer of the tendon, whilst the fibres above these reach its lowest 



part. The posterior layer ascends higher on the outer bicipital 



idge than the anterior, and from its upper border a tendinous 



xpansion passes to the great tuberosity of the humerus and the 



apsular hgament of the shoulder-joint, which conceals the long 



"udon of the biceps brachii. From the lower border of the tendon 



n expansion is given to the deep fascia of the arm. 



Nerve-stipply. — (i) The external anterior thoracic from the outer 



jprd of the brachial plexus, (its fibres coming from the fifth, sixth, 



ad seventh cerviccd nerves), the branches of which nerve enter 



