Page:EB1911 - Volume 19.djvu/416

 (mesial) branch through which the skin and muscles of the back are supplied.

It will be seen from the foregoing that the thoracic nerves are almost completely segmental in their distribution, in other words, each supplies a slice of the body, but in the other regions this segmental character is masked by the development of the branchial skeleton and the limbs. In the cervical region the first cervical or suboccipital nerve comes out between the occiput and atlas and does not always have a posterior root. When it has not, it obviously can supply no skin. Its anterior primary division joins those of the second, third and fourth cervical nerves to form the cervical plexus, from which the skin of the side of the neck and lower part of the head and face are supplied by means of the small occipital, great auricular, superficial cervical, suprasternal, supraclavicular and supra-acromial nerves (see fig. 7) as well as those muscles of the neck which are not supplied by the cranial nerves. The phrenic nerve, which comes chiefly from the fourth cervical, deserves special notice because it runs down, through the thorax, to supply the greater part of the diaphragm. The explanation of this long course (see ) is that the diaphragm is formed in the neck region of the embryo. The posterior primary division of the second cervical nerve is very large, and its inner (mesial) branch is called the great occipital and supplies most of the back of the scalp (fig. 7). The fifth, sixth, seventh and eighth anterior primary divisions of the cervical nerves as well as a large part of that of the first thoracic are prolonged into the arm, and in the lower part of the neck and armpit communicate with one another to form the brachial plexus. As a general law underlies the composition of the limb plexuses it will be worth while to study the structure and distribution of this one with some little care. It will be seen from the accompanying diagram (fig. 8) that each component nerve with the exception of the first thoracic divides into an anterior (ventral) and a posterior (dorsal) division which are best spoken of as secondary divisions in order to prevent any confusion with the anterior and posterior primary divisions which all the spinal nerves undergo. In the diagram the anterior secondary divisions are white, while the posterior are shaded. It has been suggested by A. M. Paterson that the posterior secondary branches correspond with the lateral branches of the thoracic nerves already mentioned, but there are still certain difficulties to be explained before altogether accepting this. Later on in the plexus three cords are formed of which the posterior is altogether made up of the posterior secondary divisions, while the anterior secondary divisions of the fifth, sixth and seventh cervical nerves form the external cord, and those of the eighth cervical and first thoracic the inner. As a general rule the nerves which rise from the ventral secondary divisions of the limb plexuses run only to that surface of the limb which was ventral in the embryo, while the dorsal secondary divisions are confined to the original dorsal area, but, in order to apply this to the human adult, it must be realized that the limbs