414 SPECIAL HISTOLOGY. 



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

 are enclosed by a firm sheath of connective tissue the nerve-sheath, or 

 neurilemma which also sends finer prolongations into the interior of 

 the nerves, and, as in the muscles, forms boundaries to larger or smaller 

 fasciculi, as well as extremely delicate septa between the individual 

 tubules (Fig. 160). In the ultimate ramifications, where isolated primi- 

 tive fibres, or some few of them, still often re- 

 tain an external coat, the neurilemma presents 

 the aspect of a homogeneous membrane, with 

 elongated nuclei of 0*003 of a line ; and it 

 presents this character also in the smaller 

 twigs of the cutaneous and muscular nerves, 

 only that gradually the substance begins to 

 split, in a longitudinal direction, into fibres, the nuclei become longer 

 (0-005-0-008 of a line), frequently almost like those in smooth muscles, 

 and elastic fibres also make their appearance, which are not unfre- 

 quently 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 fila- 

 ments ; they still, however, exhibit, especially in the interior, immature 

 forms of connective 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 0-002-0*004 of a line, with elongated inter- 

 stices, which invests the fasciculus, and, in fact, penetrates between its 

 elements ; never, however, surrounding individual primitive fibres, but 

 only entire divisions of them. The ganglia contain a delicate capillary 

 plexus, in the form of a network, so that each nerve-cell is surrounded 

 by special vessels. The Pacinian bodies also contain vessels, which 

 even penetrate as far as the central cavity (Todd and Bowman, II. p. 

 397, Fig. 75, and p. 399, Fig. 76 ; Herbst, Tab. IV. Figs. 1 and 2). 



On the subject of the condition of the cutaneous nerves of animals, I 

 would here add a few remarks. In the skin of the tail of batrachian 

 larvae (Rana, Bvifo, Triton, Bombinator^ Alytes\ I have described the 

 very delicate ramifications and plexuses of the embryonic pale nerve- 

 fibres ; and moreover, quite evident loops of the fully formed dark nerve- 

 tubes, and isolated divisions of the latter ("Ann. des S. Nat.," 1846, 

 p. 102, pi. 6, 7). In the full-grown Frog, according to Czerm&k (Mull. 

 "Archiv," 1849, p. 252), the nerves destined to the skin, form, on its 

 inner aspect, a wide network, already described by Burdach, from which 



FiG. 160. Transverse section of the ischiatic nerve, magnified 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. 



