620 THE SYMPATHETIC NERVE. 



motor, va80-constrictor, or vaso-hypertonic nerves [also accelerator or augmentor 

 nerves of the heart]. They run in the sympathetic for a part of their course 



( 3 71). 



4. Inhibitory fibres for the blood-vessels. These are but imperfectly known. 

 They are also called vaso-dilator or vaso-hypotonic nerves ( 372). [Also inhibitory 

 nerves for the heart, which leave the spinal axis in the vagus.] 



5. Secretory fibres for the sweat-glands of the skin ( 289). For a part of their 

 course they run in the sympathetic. 



6. Trophic fibres of the tissues ( 342, I., r). 



The posterior roots contain all the sensory nerves of the whole of the skin and 

 the internal tissues, except the front part of the head, face, and the internal part of 

 the head. They also contain the tactile nerves for the areas of the skin already 

 mentioned. Stimuli which discharge reflex movements are conducted to the spinal 

 cord through the posterior roots. The sensory fibres of a mixed nerve-trunk supply 

 the cutaneous area, which is moved by those muscles (or which covers those muscles) 

 to which the same branch supplies the motor fibres. The special distribution 

 of the motor and sensory nerves of the body belongs to anatomy (figs. 429. 430, 

 437, 438). 



[Physiology of the Limb-Plexuses. The idea that the nerve-strands become rearranged in the 

 limb-plexuses so as to connect nerves derived from different parts of the spinal cord with 

 particular groups of muscles, in order to allow of "co-ordination of muscular action," does not 

 seem to be borne out by more extended observation. Herringham has shown by dissection (and 

 the same is seen in cases of paralysis of motion and sensation) that a given muscle or part of a 

 muscle, and a given spot of skin, are supplied by particular branches of individual spinal nerves 

 proceeding directly from the spinal cord. The reason that the plexuses exist is, apparently, not 

 a physiological one. Co-ordination cannot be effected in the plexus, where the axis-cylinders 

 of the nerves do not divide ; but only in the spinal cord and central nervous system, and 

 through the intervention of nerve-cells. The existence of the plexuses is due to the fact that 

 t'lnbryologically the limb consists of a flattened lappet, or bud, derived from certain somites, but 

 at first presenting no signs of segmentation, with a preaxial and a postaxial border, and outer 

 (dorsal) and inner (ventral) surfaces of skin, covering a double layer of muscle on each surface. 

 The dorsal and ventral branches of the nerves supply these respective surfaces ; and after the 

 nerves have grown out, the simple muscular strata become split up into individual muscles, 

 which contain elements derived from one or more segments represented in the primitive limb. 

 Each nerve is segmental, and, therefore, supplies a muscle derived, for example, from the 

 elements of two segments ; the nerve of distribution must contain corresponding parts of two 

 segmental nerves. The plexuses appear, therefore, from an embryological cause, and have no 

 direct physiological significance {A. M. Paterson).] 



356. THE SYMPATHETIC NERVE. [Anatomical. The sympathetic nervous system 

 contains a large number of non-medullated or Remak's fibres, and consists of a series of ganglia 

 lying on each side of the vertebral column and connected with each other by inter-ganglionic 

 fibres. The typical distribution obtains in the thoracic region, where the lateral or vertebral 

 ganglia lie close on the vertebra?. In front of this is a second series of ganglia, which do not 

 form a double line, but are connected with the former and with each other. They are the pre- 

 vertebral or collateral ganglia, e.g., semilunar, inferior mesenteric, &c, the nerves connecting 

 them with the former being called rami etferentes. From these, fibres proceed to connect them 

 with ganglia lying in or about tissues or organs the terminal ganglia (Gaskell).] 



[Each spinal nerve in this region is connected with its corresponding sympathetic ganglion 

 by the ramus communicans, which is formed by fibres both from the anterior and posterior roots 

 of a spinal nerra. It corresponds to the visceral nerve of the morphologist, and is composed of 

 two parts a white and a grey ramus. The white ramus is composed entirely of medullated 

 fibres, and coming from the anterior and posterior roots of a spinal nerve, passes into the lateral 

 and collateral ganglia. These white rami occur in the dog only from the 2nd thoracic to the 

 2nd lumbar nerve (fig. 439). Above and below this, the rami are all grey and composed of non- 

 medullated nerve-fibres {Gaskell).'] 



[In man, the four upper rami communicantes from the four upper cervical nerves all join the 

 superior cervical ganglion (fig. 428, G g, s), the 5th and 6th join the middle cervical, the 7th 

 and 8th the inferior cervical ganglion. The lowest pair of ganglia are generally united by a 

 loop on the front of the first coccygeal vertebra, and they lie in relation with the coccygeal 

 ganglion.] 



[Cephalic Portion. As the sympathetic ascends to the head it forms connections with many 



