THE SPINAL NERVES. 



617 



of connective-tissue, which sends processes into the swelling, and is continuous with the sheaths 

 of the nerve entering and leaving the ganglion (fig. 436, c). In mammals, 

 tudinal section of such a ganglion exhibits the cells 

 arranged in groups, with strands of nerve-fibres coursing 

 longitudinally between them (fig. 436, a, b). The nerve- 

 cells are usually globular in form, with a distinct capsule 

 lined with epithelium, and the cell-substance itself con- 

 tains a well-defined nucleus with a nuclear envelope and 

 a nucleolus. The capsule of the cell is continuous with 

 the sheath of Schwann of a nerve-fibre. The exact relation 

 between the nerve-fibres and the nerve-cells is difficult to 

 establish, but it is probable that each nerve-cell is con- 

 nected with one nerve-fibre, i.e., they are unipolar. In 

 the spinal ganglia of the vertebrates above fishes, and also 

 in the Gasserian ganglion, cells are found with a single 

 process or fibre attached to them, the nerve-fibre process 

 not unfrequentl} 7 coiling a few times within the capsule. 

 This process, after emerging from the capsule, becomes 

 coated with myelin, and usually soon divides at a node of 

 Ranvier (fig. 341, t). Ranvier, who first observed this 

 arrangement, describes it as a T-shaped fibre. These 

 nerve-cells with T-shaped fibres have been observed in the 

 spinal ganglia of all vertebrates above fishes, in the Gas- 

 serian and geniculate ganglia, as well as in the jugular 

 and cervical ganglia of the vagus. In fishes, the nerve- 

 cells of the spinal ganglia are bipolar (fig. 368, 4). There 

 is a rich plexus of capillaries in these ganglia, and each 

 cell is surrounded by a mesh-work of capillaries, which 

 never penetrate the cell capsules.] 



Bell's Law. Sir Charles Bell discovered (1811) 

 that the anterior roots of the spinal nerves are 

 motor, the posterior are sensory. 



Recurrent Sensibility. Magendie discovered 

 (1822) the remarkable fact that sensory fibres are 

 also present in the anterior roots, so that their 

 stimulation causes pain. This is due to the fact Longitudinal section of a spinal gan- 

 ,,. ~, r . , , . . ri ghon. a, nerve-fibre; b, nerve-cells; 



that sensory fibres pass into the anterior root alter capsule. 



the two roots have joined, and these fibres run in 



the anterior root in a centripetal direction (Schiff, CI. Bernard). The sensibility of 

 the anterior root is abolished at once by section of the posterior root. This condition 

 is called " recurrent sensibility " of the anterior root. When the sensibility of the 

 anterior root is abolished, so is the sensibility of the surface of the spinal cord in the 

 neighbourhood of the root. A long time after section of the anterior, and when the 

 degeneration phenomena have had time to develop ( 325), a few non-degenerated 

 sensory fibres are always to be found in the central stump (Schif, Vulpian). 

 Schiff found that, in cases where the motor fibres had undergone degeneration, 

 there were always non-degenerated fibres to be found in the anterior root, which 

 passed into the membranes of the spinal cord. The sensory fibres pass into the 

 motor root, either at the angle of union of the roots, or in the plexus, or in the 

 region of the peripheral terminations. Sensory fibres enter many of the branches 

 of the motor cranial nerves at their periphery, and afterwards run in a centripetal 

 direction (p. 602). Even into the trunks of sensory nerves, sensory branches of 

 other sensory nerves may enter. This explains the remarkable observation, that 

 after section of a nerve trunk (e.g., the median), its peripheral terminations still 

 retain their sensibility (Arloing and Tripier). The tissue of the motor and sensory 

 nerves, like most other tissues of the body, is provided with sensory nerves (Nervi 

 nervorum, p. 530). 



[It does not follow that section of a peripheral cutaneous nerve will cause anaesthesia in the 

 part to which it is distributed ; in fact, one of the principal nerve trunks of the brachial plexus 



Fig. 436. 



