527 



ORGANS 



While the unity of a tissue is structural, so that wherever in 

 the body it may occur it can be recognised by its appearance, 

 that of an organ is topographical and usually physiological. It 

 is a localised part of the body, and it carries out a single function, 

 or rarely two or three functions. To do this it usually contains 

 several tissues, and although one only of these may be concerned 

 in the fundamental process which the organ carries out, without 

 the other ancillary tissues the function could not be efficiently 

 performed. For example, the essential tissue of a gland is the 

 secretory epithelium, but the gland cannot function properly 

 unless it has blood vessels which bring the raw materials of the 

 secretion, and nerves which control its discharge. A gland must 

 therefore contain many tissues besides epithelium. 



SKIN 



We have already, in describing the tissues, touched on the 

 structure of various organs. A few of the more important will now 

 be considered more formally. First we may take the skin, which 

 in mammals at least is well enough developed to be looked on 

 as an organ. It separates the body from the outside world, so 

 that impermeability must be one of its properties, but this im- 

 permeability is controlled and modified by the existence of pores 

 and by modifications of the cells. The structure of skin as seen 

 in section is shown in Fig. 395. The outer part or epidermis has 

 already been described as an example of a stratified epithelium. 

 We need only add here that it is pierced by hairs and by 

 the ducts of sweat glands, and that its basal layers contain, 

 in addition to the ordinary polyhedral cells, many which are 

 branched. These are called melanocytes or dendritic cells, and it is 

 they alone which in pigmented skin form melanin. In white skin 

 the cells are present, but they are unable to make pigment. 

 Below the epidermis is the dermis or corium, which is connective 

 tissue of mesodermal origin, mainly white fibres but with some 

 elastic fibres in its outer parts. In it run blood vessels, lymphatics 

 and nerves. Below it is a layer of subcutaneous adipose tissue, 

 and below this again more connective tissue making the super- 

 ficial fascia. In places the dermis shades imperceptibly into tliis. 

 In places, as for example, the penis, scrotum and nipple, and 



