CUTANEOUS NERVES 



401 



non-medullated nerves supply the numerous vessels, the arrector pili 

 muscles, and the sweat glands; an epilamellar plexus outside of the base- 

 ment membrane sends branches through the membrane to terminate 

 in contact with the gland cells. Medullated sensory nerves end in the 

 various corpuscles already described, and in free terminations, some 

 being intraepithelial. Medullated fibers to the hairs lose their myelin 

 and form elongated free endings with terminal enlargements in contact 

 with the hyaline membrane. (The nerves to the tactile hairs of some 

 animals penetrate the hyaline membrane and terminate in tactile menisci 

 among the cells of the outer epithelial sheath.) Small, round or discoid 

 elevations of the epidermis, visible with the naked eye, occur close to the 

 hairs as they emerge from the skin, being on the side toward which the 

 hairs slope. These "hair discs" (Pinkus) are said to be abundantly sup- 

 plied with nerves. The corium beneath the nails is rich in medullated 

 nerves, the non-medullated endings of which enter the Golgi-Mazzoni 

 type of lamellar corpuscle (having a large core and few lamellae), or they 

 form knots which are without capsules. Elsewhere the skin contains 

 tactile corpuscles in its papillae and lamellar corpuscles in the subcutaneous 

 tissue, together with free endings in the corium and epidermis (as far out 

 as the stratum granulosum) . 



MAMMARY GLANDS. 



In young mammalian embryos generally, the mammary glands are 

 first indicated by a thickened line of ectoderm extending from the axilla 

 to the groin. Later much of the line disappears, leaving a succession of 

 nodular thickenings corresponding with the nipples. In some mammals 



FIG. 413. SECTION THROUGH THE MAMMARY GLAND OF AN EMBRYO OF 25 CM. 

 i. Connective tissue of the gland. (After Basch, from McMurrich.) 



this row of nipples remains, in others only the inguinal thickenings, and 

 in still others only those toward the axilla. Thus in man there is normally 

 only one nipple on each side, but structures interpreted as accessory nipples 

 are frequent; they are not always situated along the mammary line. 

 In an embryo of 25 cm. (Fig. 413) several solid cords have grown out from 

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