228 SPECIAL HISTOLOGY. 



cutis, which are found in the neighbourhood of the sebaceous 

 glands, and whose contraction is, perhaps, not inoperative 

 towards the emptying of their contents. 



§75. 



Development of the sebaceous Glands. — The first formation 

 of the sebaceous glands takes place at the end of the fourth 

 and in the fifth month, and is intimately connected with that 

 of the hair-sacs, since they make their appearance simultane- 

 ously with the hairs, or shortly after, as out-growths of the 

 hair-sacs ; whence they are not all formed at once, but those of 

 the eyebrows, forehead, &c, first, those of the extremities last. 

 The mode of their development, more precisely described, is as 

 follows : — When the rudiments of the hair-sacs have attained a 

 considerable development, and the first indication of the hair 

 is visible in them (fig. 75, A, B), there are perceptible, on their 

 outer surface, small, indistinctly-bounded papillary processes 

 (u, v), which consist of a cellular substance, solid throughout and 

 continuous with the outer root- sheath, and of a delicate in- 

 vestment, which is continuous with that of the hair-sac. These 

 processes of the external root-sheaths of the hair-sacs, as they 

 may properly be called, at first of 0'02 — O03"' length, and 

 O01 — 0*01 6'" thickness, now begin to grow in proportion to 

 the hair-sacs, become globular, and finally, Avhile they extend 

 themselves and incline obliquely towards the bottom of the sac, 

 pyriform and flask-shaped. A formation of fat in the internal 

 cells now commences (fig. 88, A), which, beginning at the 

 bottom of the pyriform processes, is continued, also, into their 

 pedicles, and finally includes the cells of the outer root-sheath, 

 until at last the fat-cells reach as far as the canal of the hair- 

 sac (fig. 88, B) . The gland and its contents are now complete, 

 and it needs only that the cells at the bottom of the gland, or 

 the glandular vesicles, should multiply, to force the sebaceous 

 cells in the duct into the hair-sac, and fullv to establish the 

 secretion. The sebaceous glands, therefore, like the sudoriparous, 

 are, at first, solid outgrowths of the Malpighian layer of the skin, 

 for which an external opening is not developed till afterwards, 

 and the first cutaneous sebaceous matter is formed by a 

 metamorphosis of the inner cells of the rudiment of the 

 gland, while the space which these cells occupied becomes 



