SEROUS SYSTEM. 263 



pathetic they are not. In the idiopathic the serum is greatly 

 altered in colour, consistence, &c.; in the sympathetic the 

 serum is healthy. 



Development of the Serous System. 



Q. How does the development of the serous tissues pro- 

 gress? 



#. . In proportion to that of the organs they cover. 



Q. Which is earliest and most rapidly developed? 



/?. The arachnoides, and hence hydrocephalic effusion 

 is so frequent in infancy: next in development is the 

 pleura, hence the frequency of pleurisy in youth. 



Q. What alteration in form often takes place in diseases 

 of the serous tissues? 



Jl. They become in some cases much thicker, and in 

 others thinner than in their healthy state. 



Q. Why is it of practical importance to know the dis- 

 placements of the serous membranes? 



/?. Because in these displacements there is a change in 

 the relations of parts. 



Q. What is the nucleus of the various tubercles, tu- 

 mours, &c. which are seen in such numbers on the serous 

 membranes? 



#. The nucleus is formed by a concretion of albumen 

 which becomes vascular. 



Q. In what points do hydatids and cysts differ? 



t/?. Cysts are connected with surrounding parts by cel- 

 lular substance and blood-vessels; hydatids are not. The 

 parietes of cysts have a degree of consistence which hyda- 

 tids do not exhibit. Hydatids have granular substances 

 on some part of their surface ; cysts have not. 



