THE NERVOUS SYSTEM. 471 



skin, beak, and tongue (llcrbst, "Will), and with respect to 

 which physiology is still wholly in the dark, will be found in 

 the works above quoted, and also in Reichcrt (' Bindegewebe/ 

 p. 65), Herbst (' Gott. gel. Anz./ 1848, Nos. 162, 163, 18."><) ; 

 'Nachr. v. d. Univ./ p. 204, 1851, p. 161), Will (' Sitzungsber. 

 d. Wiener Acad./ Feb., 1850), Osann (' Bericht iiber d. zoot. 

 Anst. in Wiirzb./ 1819), Strahl (Midler's 'Arch./ 1818, p. 163), 

 and Pappenheim ( ( Comptes rendus/ xxiii, p. 68). [Todd and 

 Bowman, ' Physiol. Anatomy/ Part II, p. 395, figs. 71, 75, 76; 

 and Bowman, art. 'Pacinian Bodies/ ' Cyc. of Anat. and Phys/] 



The spinal nerves, from their point of exit through the dura 

 mater, are enclosed bv a firm sheath of connective tissue — the 

 nerve-sheath, or neurilemma — which also sends finer prolonga- 

 tions into the interior of the nerves, and, as in the muscles, 

 forms boundaries to larger or smaller fasciculi, as well as ex- 

 tremely delicate septa between the individual tubules (fig. 160). 

 In the ultimate ramifications, where isolated primitive fibres, 

 or some few of them, still often retain Yig. 160. 



an external coat, the neurilemma pre- f a 



sents the aspect of a homogeneous £% 

 membrane, with elongated nuclei of 

 O'OOS'"; and it presents this cha- 

 racter also in the smaller twigs of 

 the cutaneous and muscular nerves, 

 only that gradually the substance begins to split, in a longi- 

 tudinal direction, into fibres, the nuclei become longer (0-005 

 — O'OOS'"), frequently almost like those in smooth muscles, 

 and elastic fibres also make their appearance, which are not 

 unfrequently entwined around whole fasciculi. The larger 

 nerves, lastly, present common connective tissue, with distinct 

 longitudinal fibrils, as in fibrous membranes, intermixed with 

 numerous reticulated elastic filaments ; they still, however, 

 exhibit, especially in the interior, immature forms of con- 

 nective tissue. 



All the larger nerves contain vessels, although not in great 

 number; they run principally in a longitudinal direction, and 

 form a loose plexus of minute capillaries of 0'002 — 0004"', 



Fig. 160. Transverse section of the ischiatic nerve, x 15 diam.: a, general sheath 

 of the nerve; b, neurilemma of the tertiary fasciculi; c, secondary nervous fasciculi, 

 in part with special sheaths. From the Calf. 



