334 NORMAL HISTOLOGY. 



are wanting. The periphery of the alveolus is occupied by a single, or 

 incompletely double, layer of flattened and imperfectly defined basal cells. 

 These rest immediately upon the membrana propria and are distinguished 

 by their dark cytoplasm and outwardly displaced oval nuclei. Passing 

 towards the centre of the alveolus, the next cells contain a number of 

 small oil drops which, with each successive row of cells, become larger 

 and appropriate more and more space at the expense of the protoplasmic 

 reticulum in which they are lodged. In consequence, the cells occupy- 

 ing the centre of the alveoli, which are completely filled and without a 

 lumen, contain little more than fat. As the cells are escaping from the 

 glands they lose their nuclei and individual outlines and, finally, are merged 

 as debris into the secretion, or sebum, with which the hairs and skin 

 are anointed. The necessity for new cells, created by the continual 

 destruction of the glandular elements that attends the activity of the seba- 

 ceous glands, is met by the elements recruited from the proliferating basal 

 cells, which in turn pass towards the centre of the alveolus and so displace 

 the accumulating secretion. 



The Sweat-Glands. These structures, the glandul<z sudoripara, 

 occur within the integument of all parts of the body, with the exception of 

 that covering the red margins of the lips, the inner surface of the prepuce 

 and the glans penis. They are especially numerous in the palms and soles, 

 in the former locality numbering more than 1 100 to the square centimeter, 

 and fewest on the back and buttocks, where their number is reduced to 

 about 60 to the square centimeter; their usual quota for the same area is 

 between two and three hundred. 



Modified simple tubular in type, each gland consists of two chief divi- 

 sions, the body or gland-coil, the tortuously wound tube in which secretion 

 takes place, and the excretory duct, which opens on the surface of the skin, 

 exceptionally into a hair-follicle, by a minute orifice, the sweat-pore, often 

 distinguishable with the unaided eye. 



The body of the gland, irregularly spherical or flattened, consists of 

 the windings of a single or rarely branched tube. It commonly occupies the 

 deeper part of the corium, but sometimes, as in the palm and scrotum, lies 

 within the subdermal connective tissue. The coiled portion of the gland is 

 not entirely formed by the secretory segment, since, as shown by the recon- 

 structions of Huber, about one fourth is contributed by the convolutions of 

 the first part of the duct. 



The secreting" portion of the gland-coil, called the ampulla on account 

 of its greater diameter, possesses a wall of remarkable structure. The thin 

 external sheath, composed of a layer of dense fibrous tissue and elastic fibres, 

 supports a well defined membrana propria. Immediately within the latter 

 lies a thin but compact layer of involuntary muscle, whose longitudinally dis- 

 posed spindle-shaped elements in cross-section appear as irregularly nucleated 

 cells that encircle the secreting epithelium and displace it from its customary 

 position against the basement membrane. This muscular tissue enjoys the 

 distinction, which it shares with the muscles of the iris, of being developed 

 from the ectoderm. The secreting cells constitute a single row of low col- 

 umnar epithelial elements, that lie internal to the muscle and surround the 

 relatively large lumen. Their finely granular cytoplasm contains a spherical 

 nucleus, situated near the base of the cell, and in certain of the larger glands, 

 as the axillary, includes fat droplets and pigment granules. These are liber- 

 ated with the secretion of the gland and, when present in unusual quantity, 

 account for the discoloration produced by the perspiration of certain individ- 



