DEVELOPMENT OF PERIPHERAL NERVES. 1375 



Practical Considerations. — The cervical sympathetic may be injured by deep 

 wounds of the neck, or may be compressed by tumors, abscesses or aneurisms. It 

 supplies motor fibres to the involuntary muscles of the orbit and eyelids, vasomotor 

 fibres to the face, neck and head, dilator fibres to the pupil, accelerator fibres to the 

 heart and secretory fibres to the salivary glands. If it is irritated, some or all of the 

 following symptoms will be present : the palpebral fissure will open wider, the eyes 

 will be protruded, the skin of the face and neck will be pale and cold, the pupils 

 dilated, and the sweat, nasal secretion and saliva diminished. Section or destruction 

 of the cervical sympathetic will give the opposite symptoms. 



The cervical sympathetic has been removed for epilepsy, glaucoma and exoph- 

 thalmic goitre. The greatest success has been obtained in the last condition, espe- 

 cially by Jonnesco, who advises this procedure in hysteria, chorea, and tumors of the 

 brain, as well as in the above-mentioned conditions. It may be excised through an 

 incision anterior to the sterno-mastoid, as it lies posterior to the carotid sheath 

 on the prevertebral fascia. The superior cervical ganglion is the largest and lies 

 opposite the transverse processes of the second and third vertebrae. Branches of it 

 go upward along the external and internal carotid arteries, the ascending branch 

 passing along the internal carotid artery through its bony canal in the base of the 

 skull to form the carotid and cavernous plexuses, both of which are really parts of 

 one plexus arranged around this artery. Other branches communicate with the 

 cranial nerves, the pharyngeal nerves and the superficial cervical cardiac nerve. 

 The middle cervical ganglion is the smallest, lies on the inferior thyroid artery oppo- 

 site the sixth cervical vertebra and is in danger in the ligation of that artery. The 

 inferior ganglion, intermediate in size between the other two, lies in a depression 

 between the neck of the first rib and the transverse process of the seventh cervical 

 vertebra. 



The branches of the upper four or five thoracic ganglia of the sympathetic enter 

 into the supply of the thoracic viscera, but the branches of the lower seven or eight 

 form the splanchnic nerves and go to the supply of the abdominal viscera through the 

 solar plexus and its extensions into other sympathetic plexuses of the abdomen. It 

 is of interest and importance to observe that those intercostal nerves corresponding in 

 their origin from the spinal cord with the ganglia giA'ing off the splanchnics, together 

 with the first two lumbar nerves, the ilio-hypogastric and ilio-inguinal, supply the 

 abdominal wall with motor and sensory branches. In this way the same segments 

 of the spinal cord supply the abdominal viscera as well as the skin and muscles over 

 them. A similar arrangement of the nerves is seen in the joints, where the same 

 nerves supply the skin covering the joint, the muscles which move it, and the joint 

 structures. As a result of this, when necessary, all parts of the joint act in sympa- 

 thy. In an inflammation of the joint the skin becomes sensitive, tendirig to ward ofi 

 interference, and the muscles become rigid, preventing motion and favoring rest. In 

 a similar manner the abdominal muscles become rigid to protect inflamed viscera 

 underneath, the muscles of one side only if the inflammation is localized to one side, 

 but the muscles of both sides if a general peritonitis is present. 



DEVELOPMENT OF THE PERIPHERAL NERVES. 



The manner in which the nerve-fibres composing the peripheral nervous system develop 

 from the primary cells, the neuroblasts, has been indicated in the previous sketch of their 

 histogenesis given on page loii. It remains, therefore, to describe briefly at this place the more 

 important features of their morphogenesis. The fundamental fact has been repeatedly empha- 

 sized, that efferent or motor fibres are outgrowths from neurones situated within the cerebro- 

 spinal axis, whilst all afferent or sensory fibres arise from cells placed outside this axis and 

 within the ganglia located along the course of the nerves. It is evident, furthermore, that the 

 efferent constituents of the peripheral nerves have their nuclei of origin within the spinal cord 

 or brain and grow outward, as axones, to their destinations. The afferent fibres, on the other 

 hand, proceed in both directions, the axones early growing centrally to join the nervous axis, 

 hence, having usually a short course, being represented by the entering sensory roots. The 

 dendrites grow in the opposite direction and contribute the sensory fibres that extend often to 

 remote parts of the body. Whilst in the lowest vertebrates, the amphioxus and the cyclos- 

 tomes, the ventral and dorsal roots of the spinal nerves remain distinct, in the higher types 

 they join to form the mixed nerve, which typically divides into the anterior, posterior and 



