332 THE SKIN. 



cles, becoming smaller and darker, collect together in roundish 

 heaps, which may remain in this condition, or become each 

 surrounded by a cell-wall. The cells thus produced may con- 

 tain from one to twenty blood-corpuscles in their interior. 

 These corpuscles become smaller and smaller ; exchange their 

 red for a golden yellow, brown, or black color ; and, at length 

 are converted into pigment-granules, which by degrees become 

 paler and paler, until all color is lost. The corpuscles undergo 

 these changes whether the heaps of them are enveloped by a 

 cell-wall or not. 



Besides these, its supposed direct offices, the spleen is be- 

 lieved to fulfil some purpose in regard to the portal circula- 

 tion, with which it is in close connection. From the readiness 

 with which it admits of being distended, and from the fact 

 that it is generally small while gastric digestion is going on, 

 and enlarges when that act is concluded, it is supposed to act 

 as a kind of vascular reservoir, or diverticulum to the portal 

 system, or more particularly to the vessels of the stomach. 

 That it may serve such a purpose is also made probable by the 

 enlargement which it -undergoes in certain affections of the 

 heart and liver, attended with obstruction to the passage of 

 blood through the latter organ, and by its diminution when 

 the congestion of the portal system is relieved by discharges 

 from the bowels, or by the effusion of blood into the stomach. 

 This mechanical influence on the circulation, however, can 

 hardly be supposed to be more than a very subordinate part 

 of the office of an organ of so great complexity as the spleen, 

 and containing so many other structures besides bloodvessels. 

 The same may also be said with regard to the opinion that the 

 thyroid gland is important as a diverticulum for the cerebral 

 circulation, or the thymus for the pulmonary in childhood. 

 These, like the spleen, must have peculiar and higher, though 

 as yet ill-understood, offices. 



CHAPTER XIV. 



THE SKIN AND ITS SECRETIONS. 



To complete the consideration of the processes of organic 

 life, an4 especially of those which, by separating materials 

 from the blood, maintain it in the state necessary for the 

 nutrition of the body, the structure and fuuctions of the skin 

 must be now considered : for besides the purposes which it 

 serves (1), as an internal integument for the protection of 



