318 ACTINOMYCOSIS AND ALLIED DISEASES 



embedded in the granulation tissue, but are usually surrounded 

 by a zone of softer tissue. They may be transparent or jelly- 

 like, or they may be opaque and of various colours white, 

 yellow, greenish, or almost black. The appearance depends 

 upon their age and also upon their structure, the younger colonies 

 being more or less transparent, the older ones being generally 

 opaque. Their colour is modified by the presence of pigment 

 and by degenerative change, which is usually accompanied by a 

 yellowish coloration. They are generally of soft, sometimes 

 tallow-like, consistence, though sometimes in the ox they are 

 gritty, owing to the presence of calcareous deposit. They may 

 be readily found in the pus by spreading it out in a thin layer 

 on a glass slide and holding it up to the light. They are some- 

 times described as being always of a distinctly yellow colour, 

 but this is only occasionally the case; in fact, in the human 

 subject they occur much more frequently as small specks of 

 semi-translucent appearance, and of greenish-grey tint. 



Microscopical Characters. The parasite, which is now 

 generally regarded as belonging to the streptothrix group of the 

 higher bacteria (p. 16), presents pleomorphous characters. In 

 the colonies, as they grow in the tissues, three morphological 

 elements may be described, namely, filaments, coccus-like bodies, 

 and clubs. 



1. The filaments are comparatively thin, measuring about 

 6 /A in diameter, but they are often of great length. They are 

 composed of a central protoplasm enclosed by a sheath. The 

 latter, which is most easily made out in the older filaments with 

 granular protoplasm, occasionally contains granules of dark 

 pigment. In the centre of the colony the filaments interlace 

 with one another, and form an irregular network which may be 

 loose or dense ; at the periphery they are often arranged in a 

 somewhat radiating manner, and run outwards in a wavy or even 

 spiral course. They also show true branching, a character 

 which at once distinguishes them from the ordinary bacteria. 

 Between the filaments there is a finely granular or homogeneous 

 ground substance. Most of the colonies at an early stage are 

 chiefly constituted by filaments loosely arranged ; but later, part 

 of the growth may become so dense that its structure cannot be 

 made out. This dense part, starting excentrically, may grow 

 round the colony to form a hollow sphere, from the outer 

 surface of which filaments radiate for a short distance (Fig. 91). 

 The filaments usually stain uniformly in the younger colonies, 

 but some, especially in the older colonies, may be segmented so 

 as to give the appearance of a chain of bacilli or of cocci, though 



