Physiology of Nerves and Nerve-cells. 501 



crate electricity, and presumably a much greater quantity when active. 

 Kvi-n smooth muscles act electrically the same way, but weaker, since 

 by reason of the absence of the sarcolemimi and the irregularity of the 

 distribution of the fibres, the influence is scattered and dissipated. 



The electric action continues for awhile feebly after the partial death 

 of the muscle ; that is, after the cessation of its contractile power, and 

 ceases entirely upon death stiffening. The nerves which set up action 

 in the muscles are called motor nerves, and they are directly connected 

 with the inner or anterior part of the spinal cord, called the anterior 

 horns or columns. It is shown that if these columns be destroyed or 

 disabled, or the connecting nerves be severed, the nerves first un- 

 dergo degeneration and lose their excitability, and after a time the mus- 

 cles also lose their contractility and degenerate into mere connective 

 tissue. This happens in acute or chronic anterior polio-myelitis. 

 ( Ferrier. ) Mere disuse of the muscles while remaining attached to the 

 spinal cord does not produce so sweeping a degeneration. 



The muscles, or at least their membranous covers, are furnished with 

 a net-work of non-medullated nerves which connect the muscles with the 

 posterior or .sensory horns or columns of the spinal cord. These are en- 

 tirely independent of the motor nerves, and are evidently nerves of sense. 

 No doubt they are the nerves of the muscular sense, and convey to the 

 brain sensations of the exhaustion or fatigue of the muscle or stimula- 

 tions of pressure, cramp or electrical irritations. The muscle is there- 

 fore a sensory organ for these sensations, as the skin is for heat and 

 cold.. The muscle sheltered under the skin has lost, while the exposed 

 skin has retained, the sensibility to heat and cold. 



CHAPTER LII. 



PHYSIOLOGY OF NERVES AND NERVE-CELLS. 



The finest nerve fibre is extremely small ; scarcely visible to the naked 

 eye. It consists, when all parts are present, of a core or axis, which is 

 usually flat and band-like, and a shell called the medullary sheath or 

 marrow sheath. These two are further enclosed in another covering 

 called the neurilemma or nerve coat. The axis, when seen under a 

 high magnifying power, appears striated longitudinally, as if composed 

 of still finer fibrillae. The medullary sheath has a crumpled or wrinkled 

 appearance, as if it were too big, and it contains a fluid which oozes out 

 of a cut end of it and coagulates. The brain, medulla oblongata, and 

 spinal cord, constitute the general centers of the nervous system. These 

 centers are made up of nerve fibres and other bodies called nerve cells. 

 Ganglia are masses of nerve cells. Nerve fibres, which connect the 



