74 HAND-BOOK OF PHYSIOLOGY. 



of the group receiving motor filaments from the same parts of the nerve- 

 centre, (c) Any given part, say a limb, is less dependent upon the integ- 

 rity of any one nerve, (d) A plexus is frequently the means by which 

 centripetal and centrifugal fibres are conveniently mingled for distribution, 

 as in the case of the pneumogastric nerve, which receives motor filaments, 

 near its origin, from the spinal accessory. 



As medullated nerve-fibres approach their terminations they lose their 

 medullary sheath, and consist then merely of axis-cylinder and primitive 

 sheath. They then lose also the latter, and only the axis-cylinder is left, 

 with here and there a nerve-corpuscle partly rolled around it. Finally, 

 even this investment ceases, and the axis-cylinder breaks up into its ele- 

 mentary fibrillas. 



PERIPHERAL NERVE TERMINATIONS. 



(a.) Sensory. (1.) Pacinian Corpuscles. The Pacinian bodies or 

 corpuscles (Figs. 309 and 310), named after their discoverer Pacini, are 

 little elongated oval bodies, situated on some of the cerebro-spinal and 

 sympathetic nerves, especially the cutaneous nerves of the hands and feet; 

 and on branches of the large sympathetic plexus about the abdominal aorta 

 (Kolliker). They often occur also on the nerves of the mesentery, and 

 are especially well seen in the mesentery of the cat. They have been ob- 

 served also in the pancreas, lymphatic glands and thyroid glands, as well 

 as in the penis of the cat. Each corpuscle is attached by a narrow pedicle 

 to the nerve on which it is situated, and is formed of several concentric 

 layers of fine membrane, consisting of a hyaline ground-membrane with 

 connective-tissue fibres, each layer being lined by en dot helium (Fig. 310); 

 through its pedicle passes a single nerve-fibre, which, after traversing the 

 several concentric layers and their immediate spaces, enters a central cavity, 

 and, gradually losing its dark border, and becoming smaller, terminates 

 at or near the distal end of the cavity, in a knob-like enlargement, or in a 

 bifurcation. The enlargement commonly found at the end of the fibre, 

 is said 'by Pacini to resemble a ganglion corpuscle; but this observation 

 has not been confirmed. In some cases two nerves have been seen entering 

 one Pacinian body, and in others a nerve after passing unaltered through 

 one, has been observed to terminate in a second Pacinian corpuscle. 

 The physiological import of these bodies is still obscure. Closely allied to 

 Pacinian corpuscles, except that they are smaller and longer, with a row 

 of nuclei around the central termination of the nerve in the core, are 

 corpuscles of Herlst, which have been found chiefly in the tongues of 

 ducks. The capsules are nearer together, and toward the centre the en- 

 dothelial sheath appears to be absent. 



(2.) End-bulbs are found in the conjunctiva, in the penis and clitoris, 

 in the skin, and in tendon; each is composed of a medullated nerve-fibre 



