GLANDS IO47 



sections, preferably with the microtome. These sections may be 

 mounted in weak spirit, Goadby's solution, or glycerin-jelly ; but 

 in whatever way they are mounted, they undergo a gradual change 

 by lapse of time, which renders them less fit to display the cha- 

 racteristic features of their structure. 



Structure of the Glands. The various secretions of the body (as 

 saliva, bile, urine, etc.) are formed by the instrumentality of organs 

 termed glands ; which are, for the most part, constructed on one 

 fundamental type, whatever be the nature of their product. The 

 simplest idea of a gland is that which we gain from an examination 

 of the ' follicles ' or little bags imbedded in the wall of the stomach, 

 some of which secrete mucus for the protection of its surface and 

 other gastric juice. These little bags are filled with cells of a 

 spheroidal form, which may be considered as constituting their 

 epithelial lining ; these cells, in the progress of their development, 

 draw into themselves from the blood the constituents of the par- 

 ticular product they are to secrete ; and they then seem to deliver 

 it up, either by the bursting or by the melting away of their walls, 

 so that this product may be poured forth from, the mouth of the bag 

 into the cavity in which it is wanted. The organ which is generally, 

 though by no means accurately, called the liver presents this con- 

 dition in the lowest animals wherein it is found. In many Polyzoa, 

 compound Tunicata, and Annulata the cells of this organ can be seen 

 to occupy follicles in the walls of the stomach ; in insects these 

 follicles are few in number, but are immensely elongated, so as to 

 form tubes which lie loosely within the abdominal cavity, frequently 

 making many convolutions within it, and discharge their contents 

 into the commencement of the intestinal canal ; whilst in the 

 higher Mollusca, and in Crustacea, the follicles are vastly multiplied 

 in number, and are connected with the ramifications of gland-ducts, 

 like grapes upon the stalks of their bunch, so as to form a distinct 

 mass which now becomes known as the liver. The examination of 

 the tubes of this organ, in the insect, or of 

 the follicles of the crab, which may be 

 accomplished with the utmost facility, is 

 well adapted to give an idea of the 

 essential nature of glandular structure. 

 Among vertebrated animals the salivary 

 glands, the pancreas (sweetbreads), and 

 the mammary glands are well adapted to 

 display the follicular structure (fig. 783), 

 nothing more being necessary than to FlG - 78S.-Ultimate follicles 



. , , . of mammary gland, with 



make sections ot these organs thin enough their secreting cells a a, 



to be viewed as transparent objects. The containing nuclei b b. 

 kidneys of vertebrated animals are made 



up of elongated tubes, which are straight, and are lined with a 

 pavement-epithelium in the inner or 'medullary' portion of the 

 kidney, whilst they are convoluted and filled with a spheroidal 

 epithelium in the outer or 'cortical.' Certain flask-shaped dilata- 

 tions of these tubes include curious little knots of blood-vessels, 

 which are known as the ' Malpighian bodies' of the kidney; these 



