PECTORAL REGION 5 



made. Fortunately most of the terms used, like those already 

 mentioned, refer to things which can be seen and felt ; they, 

 therefore, are easily understood. There are, however, certain 

 terms, used when branches of spinal nerves are under con- 

 sideration, which are not self-explanatory, and it is necessary, 

 therefore, that the student should possess a knowledge of the 

 terms used in connection with spinal nerves and their branches 

 before the actual work of dissection is commenced. The 

 following points should be noted: (i) Every spinal nerve is 

 attached to the spinal medulla (spinal cord) by two roots, an 

 anterior root and a posterior root. The anterior root is non- 

 ganglionated &i\<\ the posterior root is ganglionated. (2) As the 

 roots are leaving the vertebral canal, through an intervertebral 

 foramen, they unite to form a trunk. (3) Immediately after 

 its exit from the intervertebral foramen the trunk divides into 

 a posterior ramus and an anterior ramus of which the anterior 

 ramus is, with few exceptions, much the larger. (4) Each 

 posterior ramus divides into a medial branch and a lateral 

 branch. (5) Each anterior ramus divides into a lateral branch 

 and an anterior branch (Fig. 4). 1 



Every anterior root consists of nerve fibres which spring 

 from nerve cells in the spinal medulla and pass to the muscle 

 fibres of various muscles. They carry motor impulses to the 

 muscles. Each posterior root consists of nerve fibres passing 

 to and from the nerve cells of the ganglion of the posterior 

 root The posterior root fibres carry sensory impulses, such 

 as cold, heat, pain, etc. The sensory impulses pass through 

 the cells of the ganglion of the posterior root and then onwards 

 to the spinal medulla. 



The trunk of every spinal nerve, therefore, contains both 

 motor or efferent and sensory or afferent nerve fibres, and the 

 posterior and anterior rami into which it divides also contain 

 both sets of fibres. The branches of the rami may contain 

 either both sets of fibres or only one or the other set. 

 Eventually, however, the peripheral parts of the fibres con- 

 veying impulses from and those conveying impulses to the 

 spinal medulla separate from one another. The fibres which 

 convey impulses from the spinal medulla become the motor 

 nerves which end in the muscle fibres, whilst the fibres which 



1 This division is not always obvious, and in certain situations the 

 lateral and anterior branches are called, respectively, posterior and 

 anterior branches, 

 i 16 



