66 LECTURE XVIII. 



wards, exhibits the properties of completely developed 

 pus. The perfect pus of the surface gradually passes, as 

 we descend, into crude pus, the mucous, tenacious, imma- 

 ture pus of the deeper layers, and what we call matura- 

 tion depends simply upon the gradual conversion of the 

 mucous intercellular substance of the originally tenacious 

 pus, which is allied in structure to granulations, into the 

 albuminous intercellular substance of pure pus. The 

 mucus dissolves and the creamy fluid is produced. The 

 maturation is therefore essentially a softening of the inter- 

 cellular substance. So direct is the connection which sub- 

 sists between development, and retrograde metamor- 

 phosis, physiological and pathological conditions. 



In just the same manner that the cartilage-cell may 

 become a bone corpuscle, the marrow-cell also may become 

 a bone-corpuscle. In the medullary spaces of bone those 

 marrow-cells which are situated at the circumference, 

 generally assume at a later period a more oblong form, 

 and take a direction parallel to the internal surface of 

 the medullary spaces, and the medullary tissue in this 

 situation has a more fibrous appearance and has indeed 

 been regarded as a medullary membrane, but it should 

 not be separated from the marrow in the centre of the 

 spaces, and only constitutes the most compact layer of 

 the medullary tissue. Now as soon as osseous tissue is 

 about to form, the nature of the basis-substance alters. 

 It becomes firmer, more cartilaginous, and the individual 

 cells appear to lie in largish cavities. Gradually they 

 become jagged, from sending out little processes, and 

 then nothing more is required than that calcareous salts 

 should deposit themselves in the basis-substance and 

 the bone is complete. Thus here again also the osseous 

 tissue is formed by a very direct transformation ; and by 

 the deposition of one such osteoid * la}^er after another 



* Osteoid I call the tissue which, when it takes up calcareous salts, becomes bone, 



