CLASSIFICATION OF GLANDULAR ORGANS. 349 



cerning the general anatomy of the glands date from the observations of Malpighi, who 

 was the first to correct the old notion that the secretions were discharged into the glan- 

 dular organs through openings in the blood-vessels. It is evident that nothing could have 

 been known of the mechanism of secretion before the connection between the arteries 

 and veins had been ascertained, which, it will be remembered, was also discovered by 

 Malpighi. Although the ideas of Malpighi were not at first generally received, more 

 recent observations with the microscope have shown that they were in the main correct; 

 although, from the imperfection of his optical instruments, Malpighi was unable to inves- 

 tigate very thoroughly the minute structure of the glands. 



Anatomical Classification of Glandular Organs. The organs which produce the 

 different secretions are susceptible of a classification according to their anatomical pecu- 

 liarities, which greatly facilitates their study. They may be divided as follows : 



1. Secreting membranes. Examples of these are the serous and synovial membranes. 



2. Follicular glands. Examples of these are the simple mucous follicles, the follicles 

 of Lieberkiihn, and the uterine follicles. 



3. Tubular glands. Examples of these are the ceruminous glands, the sudoriparous 

 glands, and the kidneys. 



4. Racemose glands, simple and compound. Examples of the simple racemose glands 

 are the sebaceous and Meibomian glands, the tracheal glands, and the glands of Brunner. 

 Examples of the compound racemose glands are the salivary glands, the pancreas, the 

 lachrymal glands, and the mammary glands. 



5. Ductless, or blood-glands. Examples of these are the thymus, the thyroid, the 

 supra-renal capsules, and the spleen. 



The liver is a glandular organ which cannot be placed in any one of the above sub- 

 divisions, as we shall see when we come to treat specially of its anatomy. The lymphatic 

 glands and other parts connected with the lymphatic and the lacteal system are not 

 embraced in the above classification. These are sometimes called conglobate glands. 



The general structure of secreting membranes and the follicular glands is very simple. 

 The secreting parts consist of a membrane, generally homogeneous, on the secreting sur- 

 face of which are found epithelial cells, either tesselated or of the variety called glandular. 

 Beneath this membrane, ramify the blood-vessels which furnish the elements of the secre- 

 tions. The follicular glands are simply digital inversions of this structure, with rounded, 

 blind extremities, the glandular epithelium lining the follicles. 



The tubular glands have essentially the same structure as the follicles, except that the 

 tubes are long and are more or less convoluted. The more complex of these organs con- 

 tain connective tissue, blood-vessels, nerves, and lymphatics. 



The compound racemose glands are composed of branching ducts, around the extrem- 

 ities of which are arranged collections of rounded follicles, like bunches of grapes. In 

 addition to the epithelium, basement-membrane, and blood-vessels, these organs contain 

 connective tissue, fibro-plastic elements, lymphatics, involuntary muscular fibres, and 

 nerves. In the simple racemose glands the excretory duct does not branch. 



The ductless glands contain blood-vessels, lymphatics, nerves, sometimes involuntary 

 muscular fibres, fibro-plastic elements, and a peculiar structure called pulp, which is com- 

 posed of fluid with cells and occasionally with closed vesicles. These are sometimes called 

 blood-glands, because they are supposed to modify the blood as it passes through their 

 substance. 



The testicles and the ovaries are not simply glandular organs ; for, in addition to the 

 production of mucous or watery secretions, their principal function is to develop certain 

 anatomical elements, the spermatozoids and the ova. The physiology of these organs 

 will be considered in connection with the subject of generation 



Classification of the Secreted Fluids. The products of the various glands may be 



