TllMOKS. 289 



the Haversian systems are absent the tnnior is composed of 

 superimposed lamellae like the outer circumferential lamellae of 

 the shaft of a long bone. 



tioft, spongy or cancellous osteomata are surrdunded by a 

 periostoid membrane. In structure they are similar to cancellous 

 osseous tissue. The marrow spaces may be occupied bv tissue 

 that is structurally identical to red marrow or they may be filled 

 with sarcomatous tissue, fibrous connective tisssue, etc. The 

 blood vessels are usually normal in structure and their distribu- 

 tion is through Haversian canals in the hard osteoma or the 

 spaces in the soft osteoma. 



Osteomata are invariably single; do not recur when removed; 

 have no tendency to form metastases ; and hence are benign. 



They should be diiterentiated from (1) ossification of inflam- 

 matory new growths as ringbone, spavin, myositis ossificans, 

 etc.; (2) hyperplasia of osseous tissue; (15) ossification of tumor 

 tissue as fibromata, chondromata, etc. ; (4) metaplasia in which 

 osseous tissue is the end product; (5) calcification. 



GLIOMA. 



A glioma is a tumor composed of supporting cells ("neuroglia 

 cells) of the tissue of the central nervous system. Neuroglia 

 tissue occurs in two forms, as ependymal cells lining the neural 

 canal and the ventricles and as glia cells which are derived from 

 the ependymal cells and act as a supporting framework of the 

 central nervous system. Olinmata are of two types, depending 

 upon the type of cells composing them, viz., spider cell glioma 

 and mossv cell glioma. Gliomata have 1)een observed only two or 

 three times jn the domestic animals. They usually have their 

 origin in the grav matter near the central canal of the spinal 

 cord or in the grav matter of the cerebrum. They do not become 

 large and thev are usually not encapsulated. They are composed 

 of cells that are verv similar to normal neuroglia cells. The 

 glioma ceils mav be slightU- larger than neuroglia cells but they 

 have the fibre-like processes characteristic of them. 



These tumors do not form metastases but are likely to produce 

 a fatal termination by pressure upon nerve centers. 



ODONTOMA. 



Odontomata are tumors composed of dental tissue and usu- 

 ally occur in connection witii teeth, particularly the superior 

 molars. Odontomata are of frequent occurrence, the majority 

 of dental diseases in two to five-year-old horses being due to 



