276 HISTOLOGY 



posed of several layers of polygonal cells, is thicker than the epidermis. 

 Dermal papillae are present, but hairs and sweat glands are absent. 

 In the lower part of this zone there are a few isolated sebaceous glands 

 without hairs, and the epithelium is slightly cornined. Thus it gradually 

 goes over into skin, forming a true linea ano-cutanea, but this line is not 

 well marked. It has been denned as the place where the first sheaths of 

 the hairs appear. 



The skin immediately surrounding the anus forms the zona cutanea. 

 Sweat glands are absent from the region bordering on the anus, but at a 

 distance of 1.0-1.5 cm. there is an elliptical zone, 1.25-1.5 cm. wide, con- 

 taining simple tubular coiled glands, the circum-anal glands of Gay. 

 These are very similar to sweat glands but are considerably larger. 



The outer layers of the pars analis recti include a very vascular tela 

 submucosa, which contains numerous nerves and lamellar corpuscles. 

 The muscularis mucosae terminates in slender longitudinal bundles which 

 extend for varying distances into the rectal columns (forming the M. 

 dilatator ani internus of Riidinger). The circular layer of the tunica mus- 

 cularis becomes thickened at its termination, forming the M. sphincter 

 ani internus; it extends a little below the the linea sinuosa analis. Beyond 

 the internal sphincter, which is composed of smooth muscle, striated muscle 

 fibers surround the anus forming the M. sphincter ani externus. The 

 outer longitudinal layer of the tunica muscularis ends in relation with 

 connective tissue strands which diverge as they pass downward through 

 the external sphincter, to terminate in the subepithelial tissue of the zona 

 cutanea. 



LIVER. 

 DEVELOPMENT AND GENERAL STRUCTURE. 



The liver first appears in human embryos of about 2. 5 mm. as a diver ticu- 

 lum of the ventral wall of the fore-gut, near its junction with the yolk-sac. 

 If the embryo is placed in an upright position (Fig. 271, A) the liver is 

 seen to be below the heart, and between the vitelline veins as they pass from 

 the yolk-sac to their cardiac termination. The diverticulum projects into 

 a mass of mesoderm, to which His gave the old anatomical term for dia- 

 phragm, namely septum transversum. The diaphragm develops in the 

 anterior or upper part of this septum; the lower or posterior part constitutes 

 the ventral mesentery, which extends from the fore-gut to the ventral 

 body wall. The hepatic diverticulum is in the mesenteric part of the sep- 

 tum, although it is always connected with the overlying diaphragmatic 

 shelf. 



Very early the liver becomes divided into two parts, (i) the somewhat 

 rounded diverticulum proper, lined with columnar cells with pale proto- 



