THE SKIN AND ITS APPENDAGES. 1143 



inch of the integument, and are rather less numerous on the sole of the foot In 

 both of these situations the orifices of the ducts are exceedingly regular, and open 

 on the curved ridges. In other situations they are more irregularly scattered, but 

 the number m a given extent of surface presents a fairly uniform average. In the 

 neck and back they are least numerous, their number amounting to 417 on the 

 square inch (Krause). Their total number is estimated by the same writer at 

 2,381,248, and, supposing the aperture of each gland to represent a surface of A 

 of a line in diameter, he calculates that the whole of these glands would present 

 an evaporating surface of about eight square inches. Each gland consists of a 

 single tube intricately convoluted, terminating .at one end by a blind extremity, 

 and opening at the other end upon the surface of the skin. In the larger glands 

 this single duct usually divides and subdivides dichotomously ; the smaller ducts 

 ultimately terminating in short csecal pouches, rarely anastomosing. The wall of 

 the duct is thick, the width of the canal rarely exceeding one-third of its diameter. 

 The tube, both in the gland and where it 

 forms the excretory duct, consists of two 

 layers an outer, formed by fine areolar 

 tissue, and an inner layer of epithelium 

 (Fig. 683). The external or fibro-cellular 

 coat is thin, continuous with the super- 

 ficial layer of the corium, and extends 

 only as high as the surface of the true 

 skin. The epithelial lining in the distal 

 part of the coiled tube of the gland proper 

 consists of a single layer of cubical epi- 

 thelium, supported on a basement-mem- 

 brane, and beneath it, between the epithe- 

 lium and the fibro-cellular coat, is a layer 

 of longitudinally or obliquely arranged 

 fibres, which are usually regarded as mus- 

 cular, though the evidence that this is so 

 not conclusive. In the duct and the 



13 



FIG. 683. Coiled tube of a sweat-gland cut in vari- 

 ous directions, a. Longitudinal section of the proxi- 

 mal part of the coiled tube. b. Transverse section of 

 the.same. c. Longitudinal section of the distal part 

 of the coiled tube. d. Transverse section of the same. 

 (From Klein and Noble Smith's Atlas of Histology.) 



proximal part of the coiled tube of the 



gland proper there are two or more layers of polyhedral cells, lined on their inter- 

 nal surface i. e., next the lumen of the tube by a delicate membrane or. cuticle, 

 and on their outer surface by a limiting membrana propria, but there are no mus- 

 cular fibres. The epithelium is continuous with the epidermis and with the delicate 

 internal cuticle of the epidermic portion of the tube. When the cuticle is carefully 

 removed from the surface of the cutis, these convoluted tubes of epithelium may 

 be drawn out and form short, thread-like processes on its under surface. 



The contents of the smaller sweat-glands are quite fluid ; but in the larger 

 glands the contents are semifluid and opaque, and contain a number of colored 

 granules and cells which appear analogous to endothelial cells. 



SEROUS MEMBRANES. 



The serous membranes form shut sacs, and may be regarded as lymph-sacs, from 

 which lymphatic vessels arise by stomata or openings between the endothelial cells. 

 (See page 1133.) The sac consists of one portion which is applied to the walls of 

 the cavity which it lines the parietal portion ; and another reflected over the sur- 

 face of the organ or organs contained in the cavity the visceral portion. Some- 

 times the sac is arranged quite simply, as the tunica vaginalis testis ; at others 

 with numerous involutions or recesses, as the peritoneum, in which, nevertheless, 

 the membrane can always be traced continuously around the whole circumference. 

 The sac is completely closed, so that no communication exists between the serous 

 cavity and the parts in its neighborhood. An apparent exception exists in the 

 peritoneum of the female ; for the Fallopian tube opens freely into the peritoneal 

 cavity in the dead subject, so that a bristle can be passed from the one into the other. 



