312 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



unequivocally indicate the production of an augmented amount of those products, 

 which we know to result from the retrograde metamorphosis of the muscular 

 substance, consequent upon its functional activity ( 52-62). 



308. Muscular tissue, properly so called, is as extra- vascular as cartilage or 

 dentine ; for its fibres are not penetrated by vessels ; and the nutriment required 

 for the growth of its contained matter must be drawn by absorption through 

 the myolemma. But the substance of Muscle, as a whole, is extremely vascu- 

 lar, the Capillary vessels being distributed in 



Fig. 101. parallel lines, united by transverse branches, in 



the minute interspaces between the fibres (Fig. 

 101); so that it is probable that there is no fibre, 

 which is not in close relation with a capillary. 

 The number of bloodvessels in a given space will 

 of course be greater, where the fibres and the 

 capillaries are both small, as in Mammals and 

 Birds, than where they are of larger diameter, 

 as in Reptiles and Fishes; and the former con- 

 dition will obviously be the one most favorable 

 CapiOary network of Muscle. to the performance of active changes between the 

 blood and the muscle. These changes consist, 



it would appear, not merely in the nutrition of the tissue ; but in the supply of 

 oxygen, which is a necessary condition of the excitement of its activity. We 

 shall hereafter see, indeed, that every muscular contraction probably involves 

 the disintegration of a certain amount of its substance, through the union of 

 oxygen, supplied by arterial blood, with its elements ; and that the great demand 

 for nutrition, which is occasioned by muscular activity, is for the purpose of 

 repairing this loss. The muscles of warm-blooded animals speedily lose their 

 irritability, after the supply of arterial blood has been suspended, either through 

 the cessation of the general circulation, or by deficient aeration of the fluid. 

 But the muscles of cold-blooded animals, which are very inferior in the energy 

 and rapidity of their action, preserve their properties for a much longer period, 

 after the deprivation of their supply of arterial blood; in accordance with the 

 general principle, that, the lower the usual amount of vital energy, the longer 

 is its persistence, after the withdrawal of the conditions on which it is depend- 

 ent. The very indisposition to a change of composition, on which the less ready 

 action depends, produces a longer retention of the power of acting. Much dis- 

 crepancy of opinion has existed amongst anatomists, as to the presence of Lym- 

 phatics in Muscular tissue. The microscopic inquiries of Prof. Kolliker incline 

 him to the opinion that the small muscles are destitute of absorbents, and that 

 the few lymphatics which seem to issue from some of the larger Muscles, belong 

 to their areolar sheath and its larger subdivisions. 



309. The Striated Muscles (excepting the Heart) are, of all the tissues except 

 the skin, those most copiously supplied with Nerves. These, like the blood- 

 vessels, lie on the outside of the Myolemma of the several fibres; and their in- 

 fluence must consequently be excited through it. The general arrangement of 

 these nerves is shown in Fig. 102. Their ultimate fibres or tubes cannot be 

 said to terminate anywhere in the Muscular substance ; for, after issuing from 

 the trunks, they form a series of loops, which return either to the same trunk, 

 or to an adjacent one. The occasional appearance of a termination to a nervous 

 fibril is caused by its dipping down between the muscular fibres, to pass towards 

 another stratum. 1 The nerves are almost exclusively of the motor kind; but a 



1 A different method of termination has been discovered by Prof. Wagner in the muscu- 

 lar nerves of the frog, and by Profs. Miiller and Briicke in those of the pike ; the ultimate 

 nerve-fibres themselves undergoing subdivision, and ending by exquisitely fine free extre- 



