402 STRUCTURE OF SECRETING GLANDS. 



propriety, lined by epithelium. By the multiplication and elongation of the tubular 

 branches, a vast extent of secreting surface is obtained, the tubes being partly coiled 

 up into a compact mass, which is traversed and held together by blood-vessels, and 

 sometimes also divided into lobules and supported, as in the testis, by fibrous parti- 

 tions, derived from the inclosing capsule of the gland. In consequence of their 

 intricately involved arrangement, it is sometimes difficult to find out how the 

 tubular ducts are disposed at their extremities. In the testis some are free, and 

 simply closed without dilatation, and others anastomose with neighbouring tubes, 

 joining with them in the form of loops ; in the kidney, little round tufts of fine 

 blood-vessels project into terminal dilatations of the ducts, but without piercing the 

 basement membrane. 



Certain glands do not precisely agree in structure with either of the ab'ove 

 classes of compound glands. One of these is the adult mammalian liver. Its 

 secreting cells are collected into small polyhedral masses termed the hepatic lobules, 

 pervaded by a network of capillary blood-vessels ; and the ducts begin within the 

 lobules, in the form of a network of fine channels which run between the sides 

 of contiguous cells. At an early stage of development the liver has, however, 

 a distinctly tubular character, a condition which is permanent in most of the lower 

 vertebrata. Another is the ovary, the alveoli of which are isolated and unprovided^ 

 with ducts. They are known as the Graafian follicles. When distended by secre- 

 tion they burst, and the secretion escapes, carrying the ovum along with it. They 

 are originally developed as tubules filled with epithelium (" egg-tubes " of Pfliiger). 



The thyroid gland is another body which in the adult consists of isolated 

 vesicles, but in the embryo has the form of a ramified tubular gland. The same 

 remark applies to the anterior lobe of the pituitary body. 



Besides blood-vessels, the glands are furnished with lymphatics, which in most 

 compound glands proceed from interstitial lymphatic spaces which surround the 

 alveoli. Branches of nerves have also been followed for some way into these 

 organs ; and that an influence is exerted on secreting organs through the 

 medium of the nervous system, is shown by the fact, that the flow of secretion 

 in several glands is affected by mental emotions, and that the flow of secre- 

 tion from many glands may be brought on by direct or reflex stimulation of 

 their nerves. In some cases also an increased accumulation of the materials of 

 secretion within the cells, may thus be produced. Fine non-medullated nerve-fibres 

 have in several instances been described as entering between the cells of the alveoli ; 

 and in the salivary glands, Pfliiger has affirmed a direct passage of nerve-fibres, 

 both medullated and non-medullated, into the secreting cells. His observations, 

 however, have not been confirmed by other inquirers, although Kupffer has described 

 a similar connection between nerve-fibres and secreting cells in the salivary glands 

 of insects. 



Uniting the several parts of a compound gland is a certain amount of inter- 

 stitial connective tissue, which varies in character in different glands, being in some 

 more fibrous, in others more cellular, and in others again being represented by 

 retiform tissue. 



Some glands have a special envelope, as in the case of the kidney and testis. 



The ducts of glands ultimately open into cavities lined by mucous membrane, 

 or upon the surface of the skin. They are sometimes provided with a reservoir, in 

 which the secretion is collected, to be discharged at intervals. The reservoir of the 

 urine receives the whole of the secreted fluid ; in the gall-bladder, on the other 

 hand, only a part of the bile is collected. The vesicula3 seminales afford another 

 example of these appended reservoirs. The ducts are constructed of a basement- 

 membrane and lining of epithelium, and in their smaller divisions there is nothing 

 more ; but in the larger branches and trunks, a coat, composed of connective tissue, 



