150 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cle takes during rest is for the most part stored as glycogen. 1 Although sugar 

 is considered a source of muscle-energy, the exact way in which it is employed 

 is doubtful. 



Inorganic Constituents of Muscle. Amongst the bases, potassium has the 

 greatest prominence, and sodium next ; magnesium, calcium, and small amounts 

 of iron are also found. Of the acids, phosphoric is present in the largest quan- 

 tities. 



Gases of Muscle. No free oxygen can be extracted, but carbon dioxide 

 may be obtained, in part free and in part in combination. A little nitrogen 

 can also be extracted. The amount of carbonic acid varies greatly with the con- 

 dition of the muscle ; for instance, it is much increased by muscle work. Mus- 

 cles take up oxygen from the blood freely, especially when active, and 

 when removed from the body may absorb small amounts from the air. 

 More oxygen is taken up by the muscle during rest than is liberated as 

 carbon dioxide, but during action the reverse is the case. 2 Oxygen is not 

 retained as free oxygen, but is stored in some combination more stable than 

 oxyha3moglobin. It is by virtue of the combined oxygen that the muscle is 

 enabled to do its work, but the process is not one of simple oxidation. That 

 muscles hold oxygen in available combinations was shown by Hermann, who 

 ascertained that a muscle can contract hundreds of times in an atmosphere 

 free from oxygen, and produce water and carbon dioxide. 



II. CHEMISTRY OF NERVES. 



Most of our ideas concerning the chemistry of nerves are based on analysis 

 of the white and gray matter of the central nervous system. The white matter 

 is largely made up of fibres and supporting tissue and the gray matter of nerve- 

 cells. The peripheral nerve-fibres are simply a continuation of the structures 

 in the central nervous system ; the active part of the fibre, the axis-cylinder, is 

 an outgrowth of the cytoplasm of a nerve-cell, and the surrounding medullary 

 sheath a continuation of the material which sheaths the axis-cylinder while in 

 the brain and cord. It is probable, therefore, that the chemistry of the axis- 

 cylinder approaches to that of the nerve-cell of which it is a branch, and the 

 chemistry of the medullary substance is the same outside as inside the central 

 nervous system. 



The white matter of the brain of the ox, which is largely made up of nerve- 

 fibres, is composed of about 70 parts water and 30 parts solids, about one-half 

 the latter being cholesterin, about a quarter proteids and connective-tissue sub- 

 stance, and about a quarter complex fatty bodies, neuro-keratin, salts, chiefly 

 potassium salts and phosphates, and traces of xanthin, hypoxanthin, etc. 



The nerve-fibre has a delicate sheath, the neurilemma, the exact constitution* 



1 Comptes rendus de la Soeiete de. Biologic-, 1886, civ. 



2 Ludwig und Sczelkow : Sitzungsberichte den k. Akad. Wien, 1862, Bd. xlv. Abthl. 1 ; and 

 Ludwig und Schmidt : Sitzungsberichte den math.-phys. Ctasse d. k. Sachs. Gesellschaft der Wissen- 

 seha/t. 1868, Bd. xx. ; Regnault and Reiset: Annales de Chimie et de Physique, 1849, 3 me se>., 

 xxvi. ; Pfliiger : Pfluger's Archiv, 1872, vi. ; and others. 



