74 ANIMAL BIOLOGY. [Part I. 



with dilute acetic acid. They are embedded in a cementing 

 substance. 



8. Nerve. The nervous elements are divided into (a) nerve 

 fibres, and (b) nerve cells. 



(a.) Nerve fibres. Such a nerve as the sciatic consists of a 

 number of nerve bundles bound together. Each nerve bundle 

 (of which three are shown in 26, iv.) is surrounded by a fibrous 

 sheath of connective tissue (perineurium), and is bound to other 

 bundles by bands of similar tissue (epineurium), while the fibres 

 within the bundle are bound together by more delicate endo- 

 neurium. Within the bundle are a number of dots, many or 

 most of them surrounded by a transparent ring (26, v.). Nerve 

 fibres isolated at once and examined in normal saline appear 

 as delicate glassy rods, without apparent external membrane, 

 but with faint indications of a central thread. Under the 

 influence of re-agents, however, we see the nerve fibre com- 

 posed of three distinct parts a central thread, the axis fibre, 

 around this a fatty medullary sheath, and external to this 

 again a primitive sheath. Chloroform dissolves the medullary 

 sheath, and makes clearly visible the axis fibre, as in 26, vi. 

 Osmic acid stains the medullary sheath black. This re -agent 

 makes obvious certain breaks in the continuity of the medulla, 

 the primary nodes (nodes of Ranvier) of which one is shown in 

 26, vii. By the subsequent addition of a staining fluid the 

 nerve corpuscles (26, viii.) in or attached to the primitive sheath 

 are rendered clearly visible. There is generally one (sometimes 

 more) such nerve corpuscles between two primary nodes. Besides 

 the primary nodes there are also much more numerous and less 

 pronounced slanting breaks in the continuity of the medulla, 

 constituting secondary nodes, and giving to this sheath an im- 

 bricated appearance. 



Near the peripheral termination of a nerve fibre the axis fibre 

 loses its medullary sheath, and becomes a non-medullated nerve- 

 fibre (Remak's fibre). The dots (in 26, v.) not surrounded by a 

 clear ring are such fibres, the others being medullated. The 

 olfactory nerve-fibres are of this non-medullated type, as are also 



