148 THE HUMAN BODY 



tween the fifth and sixth thoracic vertebrae is the '"fifth thoracic" 

 nerve, and that between the last thoracic and first lumbar verte- 

 brae, the "twelfth thoracic." In the cervical region, however, 

 this rule is not adhered to. The nerve passing out between the 

 occipital bone and the atlas is called the "first cervical" nerve, 

 that between the atlas and axis the second, and so on; that be- 

 tween seventh cervical and first thoracic vertebrae being the 

 "eighth cervical" nerve. The thirty-one pairs of spinal nerves 

 are then thus distributed : 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 

 and 1 coccygeal; the latter passing out between the sacrum and 

 coccyx. Since the spinal cord ends opposite the upper lumbar 

 vertebrae while the sacral and coccygeal nerves pass out from the 

 neural canal much farther back, it is clear that the roots of those 

 nerves, on their way to unite in the foramina of exit and form 

 nerve-trunks, must run obliquely backwards in the spinal canal for 

 a considerable distance. One finds in fact the neural canal in the 

 lumbar and sacral regions, behind the point where the spinal cord 

 has tapered off to form the filum terminate, occupied chiefly by a 

 great bunch of nerve-roots forming the so-called "horse's tail" or 

 cauda equina. 



Plexuses. Very frequently several neighboring nerve-trunks 

 send off communicating branches to one another, each branch 

 carrying fibers from one trunk to the other. Such networks are 

 called plexuses (Fig. 61), and through the interchanges taking 

 place in them it often happens that the distal branches of a nerve- 

 trunk contain fibers which it does not possess as it leaves the 

 central nervous system. 



Distribution of the Spinal Nerves. It would be out of place 

 here to go into detail as to the exact portions of the Body sup- 

 plied by each spinal nerve, but the following general statements 

 may be made. The ventral primary branches of the first four 

 cervical nerves form on each side the cervical plexus (Fig. 61) 

 from which branches are supplied to the muscles and skin of the 

 neck: also to the outer ear and the back part of the scalp. The 

 ventral primary branches of the remaining cervical nerves and 

 the first dorsal form the brachial plexus, from which the upper 

 limb is supplied. The roots of the trunks which form this plexus 

 arise from the cervical enlargement of the spinal cord. 



From the fourth and fifth cervical nerves on each side, small 



