GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 81 



the nerve. In the case of plant-cells, and of certain forms of mus- 

 cle-cells, about which there is a more or less definite wall or sheath, there 

 are little bridges of protoplasm binding the cells together. For example, 

 Engelmann describes the muscle of the intestine-- of the fly as composed of 

 striated cells, sheathed by sarcolemma, excepl where bound together by little 

 branches of sarcoplasma, which may ad as conducting wires between the cells. 



There are certain cells, however, which have been supposed to be exceptions 

 to the rule that protoplasmic continuity is essential to conduction. The stri- 

 ated muscle-fibres of the heart are quite different from those of ordinary 

 skeletal muscles, physiologically as well as anatomically. They are stumpy, 

 quadrangular cells, which arc not known to have a sarcolemma, and which 

 are united not only by their broad ends, but by lateral branches. Engelmann 

 and lately Porter 1 and others have concluded that conduction take- place in 

 the heart from cell to cell, without the intervention of nerves, and may 

 occur in all directions. This question is considered at length in the section 

 on the conduction of excitation in the heart. 



The cells of the contractile substance of some of the medusae (as Aurelia), 

 have been supposed to communicate by contiguity rather than by continuity. 

 The same has been thought to be the case with many forms of unstriated 

 muscle-tissue; 2 moreover, there are groups of ciliated cells, the members of 

 which act in unison although they have not been found to be connected either 

 directly or by nerves. These cells have apparently no membranous covering, 

 and though living as independent units, are so related that a condition of 

 activity excited in one seems to be transmitted to the resl by means of contact, 

 or through the mediation of cement-sul stance. 



From what has been said it will be seen that protoplasmic continuity 

 ensures free communication between different cells; that protoplasmic con- 

 tiguity, either directly or through the mediation of the cement-substance, may 

 possibly permit of conduction ; but that normally the intervention of a dif- 

 ferent tissue, even as deli, ate as the sarcolemma, suffices to cause complete 

 isolation of the cell from its neighbors. Under normal conditions there may 

 be a spread of excitation from muscle-fibre to muscle-fibre, even in the skel- 

 etal muscles. Kuhne's experiment with the sartorius muscle of the frog, 



described on page 45, gives a g 1 proof that the activity of a striated 



muscle-fibre is not normally transmitted to its neighbors ; nevertheless, 

 Kiihne 3 has found that if the extremities of two sartorius muscles be 

 pressed firmly together by a suitable clamp, care being taken that the 

 pressure shall not be enough to destroy the physiological activity of the 

 protoplasm, excitation of one muscle may cause contraction of the other. 

 A satisfactory explanation is lacking. 



Biedermann ' reported that when a frog's muscle was partly dried a -light 



1 Porter: Journal of Experimental Medicine, 1897, ii. p. 891 ; American Jon,,,,,! of Physi- 

 ology, 1899, ii. i». 127. ' Engelmann : Pfttger'i Arch,,; is:i. Bd iv. 

 ' * Kiihne: Untersnchin>,en am der phy iohgUchen hutttvte in Heidelberg, 1880, Bd. 3, S. 1. 

 * Biedermann: Berichte <!>;• ]\'i,„a- Abulrmic, 1888, Bd. 97, Al.tli. 3, B. 1 !•">. 

 Vol. II -6 



