CONSTRUCTION OF NERVES. 365 



The nerve-fibres have no inosculations, but pursue an uninterrupted 

 course from their central to their peripheral termination. In some instances 

 they return after a short curve to the cerebro-spinal centre, as, for example, 

 in the posterior part of the optic commissure, in the two roots of each 

 spinal nerve, and in the loop formed between the descendens noni and 

 the Dipper cervical nerves. In these cases, the fibres form a simple arch, 

 both extremities of the arch maintaining a communication with the cerebro- 

 spinal axis. In another instance, the direction of the curve is reversed, 

 the centre of the arch being in the anterior part of the optic commissure, 

 and the extremities in the retina. 



The communications which take place between nerves are termed 

 plexuses. These plexuses are sometimes formed by the trunks of the 

 nerves, as, the cervical, brachial, and lumbar ; and sometimes by the fas- 

 ciculi, as in the terminal plexuses at the periphery of the body and at the 

 surface of the brain. The nerve-fibres in the spinal cord and Central parts 

 of the brain also form a close and plexiform interlacement with each other. 

 In the construction of the larger plexuses there is a free interchange of 

 fasciculi, and in the terminal plexuses a similar interchange of smaller fas- 

 ciculi and primitive fibres. It is from the terminal plexuses that the nerve- 

 fibres pass off to form their terminal loops. 



The general mode of termination of nerve-fibres is by loops. There 

 exists, however, an exception to this rule in the instance of the Pacinian* 

 corpuscles. These corpuscles are minute bodies, about a line in length, 

 of an oval, oblong, or spheroidal shape, and smooth and glistening aspect, 

 connected with the terminal nerve-fibres of the digital branches of the 

 nerves distributed to the hands and feet. As many as two or three hun- 

 dred are met with in a single hand. They are also found, but less nume- 

 rously, on the terminal fibres of other sensitive nerves, and on the fibres 

 of the sympathetic plexuses of the mesentery, of the meso-colon, and of 

 the pancreas. They occur singly and in groups of two or three, and are 

 each connected with the nerve by means of a short pedicle which projects 

 into the corpuscle, and forms a conical process in its interior. The Paci- 

 nian corpuscle and its pedicle are composed of about fifty thin and mem- 

 branous tunics, which are closely adherent in the latter, but are separated 

 in the corpuscle by an albuminous fluid, and towards the free end of the 

 corpuscle, the tunics are connected by imperfect septa. The central 

 tunic or capsule is also filled with fluid, and into this fluid the axis cylin- 

 der of a primitive nervous fibril, derived from the nerve and continued 

 through the centre of the peduncle, is prolonged. According to Henle 

 and Kolliker, this nervous fibril generally terminates in a small rounded 

 enlargement ; at other times it bifurcates and forms two rounded heads, 

 and occasionally escapes from the corpuscle at its free end. Each of the 

 tunics of the Pacinian body is composed of fibres which have a circular 

 disposition on its external surface, and are arranged in a longitudinal 

 direction within. It is these fibres which give to the corpuscle its glisten- 

 ing appearance. The Pacinian corpuscles are first perceptible during the 

 sixth month of foetal life. 



On certain of the nerves of the body, for example, on the posterior roots 

 of the cranial and spinal nerves, and particularly on the sympathetic, are 

 situated enlargements which are termed ganglia. Ganglia are greyish in 



Discovered and described by Pacini in 1831; described more particularly in 1840. 

 and by Henle and Koliiker in 1844. 



31* 



