994 SPECIAL PHYSIOLOGY. 



Vascular or Ductless Grlands. 



The several organs thus grouped together, arise from masses of primitive 

 embryonic cells and blastema, which appear in the situations already described 

 with their development as organs. The closed sacs of the lingual, tonsillar, 

 pharyngeal, gastric, and solitary and agminated intestinal glands, and also the 

 closed sacs or Malpighian corpuscles of the spleen, are developed by the multi- 

 plication of cells or cell-nuclei, of which the outer ones form a membranous 

 envelope, and the inner ones the special pulp with its traversing bloodvessels . 

 The cells of the thyroid body, and those of the suprarenal body, originate also 

 by cell-growth, which is most readily observed in these peculiar organs. The 

 new cells of the thyroid re said to be formed by a process of budding or pro- 

 trusion, and subsequent constriction and separation. The parenchyma of the 

 spleen, the thick walls of the recesses of the thymus, as well as its fluid contents, 

 and lastly the pituitary body, are formed of gymnoplasts, nuclei, and a matrix. 



Secreting Membranes and Grlands. 



The subcutaneous synovial bursee, mere interspaces in the subcutaneous 

 connective tissue, probably arise, at first, by a process of softening and ab- 

 sorption of that tissue, and afterwards by an extension of their walls. In the 

 true synovial membranes, in the serous membranes, and in the mucous mem- 

 branes, the defined limiting or basemeni-membrane is developed from very fine, 

 almost homogeneous, connective tissue ; but in the glands, the well-defined 

 glassy feasemeni-membrane is supposed to be a sort of excretion from the epi- 

 thelial cells which cover the surface. The origin of the glands, as organs, has 

 already been described. They commence as masses of nucleated cells, evi- 

 dently destined to be epithelial ; these project into and fill up recesses in the 

 corium beneath. They either remain simple*, as in the case of the gastric tu- 

 buli, or they may extend so as to develop the most complex gland, like the 

 liver or kidney. The cavities of the ducts, which are at first solid, are formed 

 by a softening of the intercellular matrix, along certain special lines of cells. 



Epithelial and Epidermoid Tissues. 



These arise, generally, from the multiplication and metamorphosis of the 

 embryonic cells of the outer and inner germinal layer of the embryo. In the 

 case of the serous, membranes and of the synovial membranes of the joints, 

 they also originate from cells in deeper portions of the embryonic structure. 



The modifications which these cells undergo, however various, always per- 

 mit them to retain their nucleated-cell character throughout their whole ex- 

 istence. The changes of shape, structure, and contents necessary to transform 

 them into the various kinds of epithelial and epidermoid structures, can be un- 

 derstood by perusing the description of them in p. 65. Pigmentary deposits 

 may occur either in simple epithelial cells (Fig. 43), or in ramified cells (Fig. 

 122, f). 



In the many-layered epithelia, these changes may be seen at one view, all 

 occurring simultaneously (Fig. 44).. The cuticle at first cannot be distin- 

 guished from the cutis. All the epithelia, as well as the epidermis, exhibit a 

 continuous growth. The glandular epithelia show the widest departure from 

 the primitive cell-type, especially as regards the chemical composition of their 

 contents. The mode in which cilia are developed on the ciliated epithelia, is 

 not exactly known. It may be by outgrowths of the cell-wall, including pro- 

 cesses of the cell-contents, or by a fission of the substance of the cell. 



The nails are developed, not on the surface, but beneath a thin epidermic 

 covering ; the young nail consists of compressed and easily separable cells. 



The hairs appear as little black specks under the cuticle ; these are clusters 

 of colored epidermic cells of the Malpighian layer, fitting into depressions in 

 the cutis, which are lined by a basement-membrane. This rudimentary follicle 

 enlarges, and acquires its flask-shaped character ; its walls are formed by the 



