532 TEXT-BOOK OF PHYSIOLOGY 



The spinal nerves, thirty-one in number on each side, are in anatomic 

 relation with the spinal cord, and because of the fact that they pass through 

 foramina in the walls of the spinal column they are termed spinal nerves. 

 As both cranial and spinal nerves are ultimately distributed to the structures 

 of the body i.e., the general periphery they collectively constitute the 

 peripheral organs of the nerve system. 



The spinal nerves consist of two groups of nerve- fibers, a ventral and a 

 dorsal group. Though closely intermingled in the common trunk of the 

 spinal nerve they are distinctly separated near the spinal cord. Owing to 

 their connection with the ventral and dorsal surfaces of the spinal cord they 

 have been termed respectively the ventral and dorsal roots. Peripherally 

 the ventral root fibers are distributed to skeletal muscles, glands, walls of 

 blood-vessels and walls of various viscera: the dorsal root fibers are dis- 

 tributed to skin, mucous membranes, muscles, joints, etc. 



The central organs of the nerve system are supported and protected by 

 three membranes named, in their order from without inward, the dura 

 mater, the arachnoid, and the pia mater. 



The Cerebro-spinal Fluid. The general subarachnoid space, as well 

 as certain cavities within the encephalon, contain a clear transparent 

 fluid, termed the cerebro-spinal fluid. This fluid has an alkaline re- 

 action and a specific gravity of 1.007 or 1.008. It is composed of water, 

 a small amount of protein, a copper reducing substance which is now 

 believed to be glucose and inorganic salts similar to those present in 

 the blood plasma. The amount of fluid has been estimated at from 60 

 to 80 c.c. The amount, however, varies with the activity of the organ 

 by which it is produced. 



The encephalo-spinal or cerebro-spinal fluid is now regarded as a 

 product of the secretory activity of the epithelial cells covering the villous- 

 like processes of the blood-vessels of the choroid plexus. These cells 

 may therefore be regarded as an everted gland, to which the term choroid 

 gland has been given. This gland may be excited to increased activity 

 by the injection of a saline extract of the gland itself. The chemic 

 nature of the excitatory substance, however, is unknown. 



After being secreted the fluid passes into the lateral ventricles, the 

 third ventricle, through the aqueduct of Sylvius into the fourth ventricle 

 and the central canal of the spinal cord; from this common cavity the 

 fluid flows through openings in the pia mater (the foramina of Magendie, 

 Key and Retzius) or through permeable areas in the pia mater into the 

 general sub-arachnoid space. 



Experimental results indicate that the cerebro-spinal fluid is being 

 constantly secreted and as constantly being discharged, though it is prob- 

 able that the former process exceeds the latter since the fluid exerts at all 

 times a pressure, in the dog, of about 200 mm. of water. This pressure is 

 not due to transmitted arterial pressure, though perhaps modified by it, 

 but to the activity of the choroidal epithelium. During its stay in the 

 subarachnoid spaces it penetrates the perivascular and perineural spaces 

 of the cranial nerves at least, and thus directly bathes the neural tissues 

 of the brain and spinal cord. 



The exit of the fluid is by way of the arachnoid villi or the Pacchionian 

 bodies. These are small projections of the arachnoid membrane, which in 



