772 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



organs of conduction, and the nerve-cells, in which each nerve at each 

 end terminates, are for receiving and liberating impulses. 



Nerves may be of two kinds: afferent or centripetal nerves, which 

 are concerned in the carrying of impulses from the exterior to the cen- 

 tral organs ; such nerves are frequently spoken of as sensory nerves : 

 and efferent or centrifugal, which carry impulses from the central 

 portions of the nervous system to the exterior ; such nerves are motor 

 nerves. 



These two distinctions between nerves are not based on an}- differ- 

 ence in anatomical structure, but are simply functional differences, since 

 it has been found by experiment that nerves ma} T carry impulses in 

 either direction, and by dividing a motor and sensory nerve and con- 

 necting the divided extremities of the one with the other the sensory 

 nerve, after union has taken place, may now carry motor impulses and 

 the motor sensory impulses. 



The essential part of the nerve-trunk is the so-called axis cylinder, 

 which is composed of a thin filament of undifferentiated protoplasm in 

 no way different, as far as ma}' be determined, from that found in other 

 examples of free protoplasm. This protoplasmic centre, which is com- 

 posed of a number of fine fibrils and constitutes the axis cylinder, is 

 always covered by a thin, transparent membrane, which is termed the 

 primitive sheath. In many instances this is the only covering to the 

 ultimate fibrils of the nerve, such nerves being called non-medullated 

 nerve-fibres; in others, which are called medullated nerves, within this 

 primitive sheath, and surrounding directly the fibrils of the axis cjdin- 

 der, is found a thick layer of double refractive substance, which is termed 

 the medullary sheath or white substance of Schwann (Fig. 329). 



Each nerve-trunk consists of bundles of nerve-fibres held together 

 by fibrous connective tissue called the epineurium, in which are the 

 blood-vessels with which the nerve-trunk is supplied, lymphatics, and 

 numerous fatty cells. The neurilemma closely resembles sarcolemma in 

 its character; when subjected to long boiling botli yield gelatin. 



Ganglioiiic cells or nerve-corpuscles vary greatly in size. They may 

 be spherical, ovoid, pyramidal, or of other shapes, and send off usually 

 numerous branched processes, which serve to characterize the cells as 

 multipolar nerve-cells. No cell-membrane is to be detected, but the gan- 

 glia are of soft consistence, containing numerous granules and pigment 

 matter. 



The nucleus is ordinarily well developed and is disproportionate^ 

 large to the size of the cell. Two nucleoli are nearly always present. 

 One of the processes of the ganglion is always unbranched and forms the 

 axis cylinder of the nerve originating or terminating in such a nerve-cell. 

 A nerve, therefore, may be regarded simply as a process of the nerve- 



