346 MANUAL OF HISTOLOGY. 



327), from the contents of which the materials of the secretion of our 

 or^an are abstracted. 



0f the three requisites of a gland the blood-vessels and cells are never 

 absent, and the homogeneous membrane only rarely so. 



Besides these, we have to take into account the nerves distributed to 

 the or<*an the lymphatics, connective-tissue, and at times also muscular 

 envelopes'; and, finally, as a frequent occurrence, a special and often toler- 

 ably complicated excretory duct. 



194. 



The membrana propria, when such a structure exists, presents itself in 

 the form of a homogeneous envelope, at one time immeasurably thin, at 

 another thickened frequently up to O'OOll, or more rarely 0-0023 mm. 

 It is often mixed with or enveloped in an extra layer of connective-tissue 

 until a tunic of '0045-0 '0090 mm. results. As an 

 exception, we may perceive between these two strata 

 a layer of unstriped muscle, as in the large sweat 

 glands of the axilla. At times, also, as, for instance, 

 in the sebaceous glands, we find the membrana pro- 

 pria replaced by an undeveloped connective-tissue. 

 In other cases there appears (fig. 329) imbedded in 

 it a web of flattened connective-tissue cells contain- 

 ing nuclei (parotid, submaxillary, and lachrymal 

 gland). 



The structureless membrana propria manifests 

 further a considerable amount of distensibility and 

 strength, and likewise of power of resistance to the 

 Fig. 329. Web of flat action of weak alkaline solutions and acids, so that 



stellate connective-tis- . , , ,. . , j & v *. 



sue cells isolated by these may be made use of with good etiect lor its 

 membrma ^opria. demonstration. At present we are not acquainted 

 From the submaxiiiary with its chemical constitution; it is probably formed 



gland c dog, aft ^ many ^^ Qf ^^ 8ubstance closely allied to 



elastin. 



From an anatomical point of view, this covering may be regarded as 

 determining the form of the organ as already mentioned, physiologically 

 it serves for the filtration and transudation of the plasma. In respect to 

 the histology of the structure, it has been supposed to be a substratum 

 secreted by the first aggregations of rudimentary gland cells on their 

 exterior, and hardened in that situation. This process was looked upon 

 as having taken place at an early period in existence, the membrane out- 

 living many generations of gland cells. But another recent view appears 

 to us far more worthy of acceptation, namely, that the gland-membrane is 

 only the transformed, and more or less independent limiting layer of the 

 surrounding connective-tissue, and represents therefore a contiguous por- 

 tion of the middle germinal plate. This theory offers an easy explana- 

 tion for the presence or absence of the membrana propria. It seems, 

 moreover, to be a characteristic of the gland cells, in contradistinction to 

 other cellular elements of the body, that they do not generate externally 

 definite formed products. 



The shapes under which the membrana propria or limiting layer of con- 

 nective-tissue presents itself to us, are, as has been already remarked, 

 very various. Three varieties may be generally recognised, and corres- 



