20 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



those of the amoeba. 1 The property of contractility is possessed by a vast 

 variety of unicellular structures in lower forms of animal life. Another 

 example is the Vorticella (Fig. 2). 



The vorticella, like the amoeba, is a littl" animal which, although consisting 

 of a single cell, possesses within its microscopic form all the physiological prop- 

 erties essential to life and the perpetuation of its species. It consists of a bell, 

 with ciliated margin, borne upon a contractile stalk. If touched with a 

 hair, or jarred, the cell rapidly contracts; the edge of the bell is drawn in so 

 as to make the body nearly spherical, and the stalk is thrown into a spiral 

 and drags the body back toward the point of attachment. The contraction is 

 rapid ; the relaxation, which comes when the irritation ceases, is gradual. An 

 interesting account of the movements of Vorticella gracilis is given by Hodge 

 and Aikins 2 under the title of " The Daily Life of a Protozoan." 



Other examples of contractile power possessed by apparently simple organ- 

 isms are to be found in the tentacles of Actinias, the surface sarcode of sponges, 

 the chromatoblasts of Pleuroneetida?, 3 which are controlled by nerves and 

 under the influence of light and darkness change their size and so alter the 

 color of the skin, and the vast variety of ciliated forms, including spermatozoa, 

 and some of the cells of mucous membranes. 4 



Irritability. — We have thus far referred to but one of the vital properties 

 of protoplasm, viz. contractility. Another property intimately associated with 

 it is irritability. Irritability is the property of living protoplasm which causes 

 it to undergo characteristic chemical and physical changes when subjected to 

 certain external influences called irritants. Muscle protoplasm is very irri- 

 table, and is easily excited to contraction by such irritants as electric shocks, 

 mechanical blows, etc. The muscles which move the bones rarely, if ever, in 

 a normal condition, exhibit spontaneous alterations in form, and cannot be said 

 to possess automatic power. By automatism is meant that property of cell- 

 protoplasm which enables it to become active as a result of changes which 

 originate within itself, and independently of any external irritant. Examples 

 of this power may perhaps be found in the movements of ciliated organisms 

 and the infusoria. Possibly the rhythmic movements of heart muscle are of 

 this nature. Still another property of protoplasm, closely allied to contractility 

 and irritability, and possessed by niu-cle-substance, is conductivity. 



Conductivity is the property which enables a substance, when excited in 

 one part, to transmit the condition of activity throughout the irritable mate- 

 rial. For example, an external influence capable of exciting an irritable 

 muscle-fibre to contraction, although it may directly affect only a small part of 

 the fibre, may indirectly influence the whole, because the condition of activity 

 which it excites at the point of application is transmitted by the muscle-sub- 

 stance throughout the extent of the fibre. 



1 An excellent description of those movements, accompanied by illustrations, is given in 

 Quoin's Anatomy, vol. i., pt. 2. pp. 174-179. 



a Hodge and Aikins: American Journal of Psychology, 1895, vol. vi., No. 4, p. 524. 



3 Krukenberg: Vergleichend-physiologische Vbrtrdge, 1886, Bd. i. S. 271. 



* A careful study of the different forms of movement exhibited by simple organisms has 

 been made by Engelmann: Hermann's Handbuch der Physiologie, 1879. r>d i., Th. 1, IS. 344. 



