692 THE NERVOUS SYSTEM. 



3. Plexuses. The plexuses formed by the posterior rami of the upper cervical and upper 

 sacral nerves are the simplest met with in the human body. The posterior cervical plexus is 

 one from which muscular branches are supplied ; the posterior sacral plexus is mainly concerned 

 in producing cutaneous offsets. In the case of the posterior cervical plexus the loops of com- 

 munication between the first three or four cervical nerves result in the formation of a series of 

 nerves for the supply of the semispinales and other muscles, which bring into contact with 

 these muscles, simultaneously, a considerable area of the spinal medulla, and provide a combined 

 and simultaneous innervation for the -several parts of each muscle. In the case of the posterior 

 sacral plexus, the formation of loops between the nerves results in the innervation of any given 

 spot in the cutaneous area supplied from these loops by more than one spinal nerve. As has 

 been said already, the cutaneous nerves, even without the formation of plexuses, overlap in their 

 cutaneous distribution. The formation of a plexus causes a more intimate union of neighbouring 

 spinal nerves, so that stimulation of the surface affects a wider area in the spinal medulla than if 

 the nerves passed separately to it from the surface. While segmentation becomes less obvious, 

 increased co-ordination of both movement and sensation is effected. 



ANTERIOE RAMI OF THE SPINAL NERVES. 



The anterior rami (O.T. anterior primary divisions) of the spinal nerves, are, with 

 the exception of the first two cervical nerves, much larger than the correspond- 

 ing posterior -rami. Composed of elements of both posterior and anterior roots, each 

 nerve separates from the posterior ramus on emerging from the intervertebral 

 foramen, and, proceeding laterally, is distributed to structures on the lateral and 

 anterior aspects of the body, including the limbs. 



Each nerve is joined near its origin by a gray ramus communicans from the 

 corresponding sympathetic gangliated trunk ; and in the case of certain thoracic, 

 lumbar, and sacral nerves, the anterior ramus gives off a delicate bundle of fibres, 

 which forms the white ramus communicans to the sympathetic trunk. That part of 

 the spinal nerve which is distributed to the body wall and limbs may be termed 

 somatic ; the small white ramus communicans, innervating structures in the 

 splanchnic area, may be termed the visceral or splanchnic part of the spinal nerve. 



The anterior rami of the spinal nerves are distributed in a regular segmental 

 manner only in certain cases. Except in the case of the thoracic nerves, the j 

 anterior rami combine to form the three great plexuses cervical, brachial, and j 

 lumbo-sacral and their arrangement and distribution is rendered exceedingly j 

 complex. 



A thoracic nerve, such as the fifth or sixth, may be regarded as a type to I 

 illustrate the mode of distribution of the anterior rami of the spinal nerves I 

 (Fig. 606, p. 686). It occupies an intercostal space; near its origin it possesses jj 

 gray and white rami communicantes ; it courses through the interval between the 

 intercostal muscles; it supplies branches to those muscles and gives off, when it 

 reaches the side of the chest, a lateral branch, which, after supplying small muscular i 

 branches, pierces the external intercostal muscle, and is distributed to an area off. 

 skin over the lateral part of the trunk, contiguous dorsally with a similar area, 

 innervated by the cutaneous branches of the posterior ramus of the same nerve, n 

 The lateral branch generally subdivides into a smaller posterior and a larger 

 anterior ramus, as it pierces the muscles clothing the wall of the chest. The : 

 main trunk of the nerve, having given off its lateral branch, then pursues itsjj 

 course obliquely forwards to the side of the sternum, where, after piercing thef. 

 pectoral muscles, it appears superficially as the terminal anterior cutaneous branch. 

 This supplies an area of skin continuous with that supplied by the anterior part off, 

 the lateral branch of the same nerve. Such a nerve thus supplies, by means oi 

 its lateral and anterior branches, an area of skin which (with the area supplied; 

 by the cutaneous branch of its posterior ramus) forms a continuous and unin-i 

 terrupted belt, extending from the median plane behind to the median plane ir 

 front. The lateral and anterior branches of the nerve innervate in their cours< 

 the intercostal and other muscles, to be afterwards mentioned in detail. 



NERVI CERVICALES. 



The anterior rami of the cervical nerves, together with parts of the first an> 

 second thoracic nerves, are distributed to the head, neck, and upper extremity 



