SYMPATHETIC NERVES TO THE HEAD. 617 



century. A discussion of the various views which have been held, even 

 of those which have been held in recent times, would take more space 

 than it is desirable to occupy here. I shall, then, in the main restrict 

 myself to stating the views which seem to me to rest on the most satis- 

 factory experimental basis. 



GENERAL ACCOUNT OF THE SYMPATHETIC NERVOUS 



SYSTEM. 



Vertebral and pre-vertebral ganglia. — The ganglia of the sym- 

 pathetic system are divided into vertebral or lateral, and pre-vertebral or 

 collateral. The vertebral ganglia are the ganglia of the sympathetic 

 chain or trunk, and I shall include in this also the superior cervical 

 ganglia. The pre-vertebral ganglia are the ganglia of the solar plexus 

 (solar ganglion, epigastric ganglion), the inferior mesenteric ganglia, and 

 some small ganglia peripherally of the main masses. I shall include 

 in this class the inferior cervical ganglion. 



The characteristic of the vertebral ganglia is, that they send nerve 

 fibres to the cerebro-spinal nerves. The superior cervical ganglion 

 sends fibres in most mammals to the first three cervical nerves — in man 

 to the first three or four — as well as to the fifth, and to some other 

 cranial nerves. And each ganglion of the sympathetic chain sends fibres 

 to one or more of the spinal nerves. The pre-vertebral ganglia do not, 

 so far as is known, send nerve fibres to spinal nerves ; they send their 

 fibres to the thoracic, abdominal, and pelvic viscera. It is true that the 

 inferior cervical ganglion is described in man and in some other mam- 

 mals as sending strands to the lower cervical nerves ; but, as there is at 

 present no experimental evidence that the inferior cervical ganglion 

 exercises any sympathetic action in the region supplied by these nerves, 

 I have provisionally included it in the class of pre-vertebral ganglia. 



We will consider successively the sympathetic nervous supply of the 

 head, of the skin, of the trunk and limbs, of the skeletal muscles, and 

 of the thoracic and abdominal viscera. 



Sympathetic Nerves to the Head. 



Effects produced by stimulating the cervical sympathetic. — 

 Stimulation of the cervical sympathetic produces on the side stimulated 

 the following effects : — 



1. Dilatation of the pupil. — This is the result of contraction of the 

 dilatator pupilke; the contractile substance probably consists, not of 

 ordinary unstriated muscle, but of the processes of the cells covering the 

 posterior surface. The dilatation is more marked in the cat and dog 

 than in the rabbit. 



2. Retraction of the nictitating membrane; the movement of the 

 nictitating membrane is stronger in the cat and dog than in the rabbit — 

 in man and ape the nictitating membrane is rudimentary. Separation 

 of the eyelids, the movement being greater in the lower than in the upper 

 eyelids. Projection of the eye and direction of its axis straight forwards. 



3. Contraction of the blood vessels of the skin and mucous membrane of 

 the head, and of the salivary and other glands. — The rapidity and extent 

 of contraction varies in different parts of the head. It is usually very 

 great in the salivary glands and in the ear ; it is especially obvious in the 

 ear of the rabbit. It is marked in the conjunctiva, the iris, and in most 



