38 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ficial layer of protoplasm of the egg and grows until it comes in contact with 

 the nearest spermatozoon." " At the moment of contact between the sperma- 

 tozoon and the egg, the outermost layer of protoplasm of the latter raises itself 

 up as a distinct membrane, which separates from the egg and prevents the 

 entrance of other spermatozoa." Some of the eggs of arthropods and other 

 forms have likewise" been observed to undergo amoeboid movements as a result 

 of the physiological stimulus given by the spermatozoon. 1 



Although irritability and contractility of the ovum have thus far beeii made 

 out in but few forms, it is probable that they play an important part in all 

 during fertilization and division. It would seem, then, that the ovum has all 

 the principal properties which we ascribe to cell-protoplasm, and that these 

 properties are inherited more or less completely developed by the many forms 

 of cells descended from it. The protoplasm of specialized cells, in spite of 

 their differences in structure, still retains its protoplasmic nature. Undoubtedly 

 structural peculiarities are intimately related to specialized functions, the 

 striped muscle, for example, is especially adapted for rapid movements, and 

 the nerve-fibre is remarkable for its power of conduction. 



Physiological methods for the examination of individual cells are as yet in 

 their infancy, and we must, for the most part, be content to study the func- 

 tional activity of cells by observing the combined action of many cells of the 

 same kind. 



B. IRRITABILITY OF MUSCLE AND NERVE. 



Irritability is the property of living protoplasm which causes it to undergo 

 characteristic physical and chemical changes when it is subjected to certain 

 influences, called irritants, or stimuli. By an irritant is meant an external influ- 

 ence which, when applied to living protoplasm, as of a nerve or muscle, excites 

 it to action. Irritants may be roughly classed as mechanical, chemical, thermal, 

 and electrical. The normal physiological stimulus is developed within some 

 of the nervous mechanisms of the body as the result of the activity of the 

 nerve-protoplasm, this having been excited as a rule by some form of irritant. 

 The degree of irritability of a given form of protoplasm is measured by the 

 amount of activity which it displays in response to a definite irritant, or by the 

 minimal amount of irritation required to excite it to action. If the irritant be 

 applied directly to a muscle, the height to which the muscle contracts and raises 

 a given weight may be taken as an indication of its activity. As the nerve 

 gives no visible evidence of activity, the effect of the irritant upon it is usually 

 estimated by the extent to which the organ stimulated by the nerve reacts ; in 

 the case of motor nerves, the strength of the contraction of the corresponding 

 muscle is taken as an index. 



To determine the exact relation of an irritant to its irritating effect we should 

 be able to accurately measure them. This we cannot do. We are unable to 

 state in irritation-units the relative value of different kinds of irritants. Even 



1 Korschelt: Zoologischer Jahrbuch, 1891, Anat. Abtheil., Bd. iv., Heft 1, p. 1. Hertwig: 

 Morphologische Jahrbuch, 1876, Bd. 1. Herbst: Bioloyische Centralblatt, 1891, xiii. p. 22. 



