2 HANDBOOK OF PHYSIOLOGY. 



under the microscope. It is this substance, too, which forms the cells, 

 of which even the most complex organism has been proved to be made 

 up and from which it has been developed. Thus, the human body can 

 be shown by dissection to consist of various dissimilar parts, bones, mus- 

 cles, brain, heart, lungs, intestines, etc., and these, on more minute 

 examination, are found to be composed of different tissues, such as epi- 

 thelial, connective, nervous, muscular, and the like. Each of these tis- 

 sues is made up of cells or of their altered equivalents. Again, we are 

 taught by Embryology, the science which treats of the growth and 

 structure of organisms from their first coming into being, that the 

 Imman body, made up of all these dissimilar structures, commences its 

 life as a minute cell or ovum about one one hundred and twentieth of an 

 inch in diameter, consisting of a spherical mass of protoplasm, in the 

 midst of which was contained a smaller spherical body or germinal vesi- 

 cle. The phenomena of life then are exhibited in cells, whether exist- 

 ing alone or developed into the organs and tissues of animals and plants. 

 It must be at once evident, therefore, 'that a correct knowledge of the 

 nature and activities of the cell, forms the very foundation of physiology. 

 Cells are, in fact, physiological no less than morphological units. 



The prime importance of the cell as an element of structure was first 

 established by the researches of Schlejden, and his conclusions, drawn 

 from the study of vegetable histology, were at once extended by 

 Schwann to the animal kingdom. The earlier observers defined a cell 

 as a more or less spherical body limited by a membrane, and containing 

 a smaller body termed a nucleus, which iti its turn incloses one or more 

 nucleoli. Such a definition applied admirably to most vegetable cells, 

 but the more extended investigation of animal tissues soon showed that 

 in many cases no limiting membrane or cell- wall could be demonstrated. 



The presence or absence of a cell-wall, therefore, was now regarded 

 as quite a secondary matter, while at the same time the cell-substance 

 came gradually to be recognized as of primary importance. Many of 

 the lower forms of animal life, e. g , the Khizopoda, were found to con- 

 sist almost entirely of matter very similar in appearance and chemical 

 composition to the cell-substance of higher forms; and this from 

 its chemical resemblance to flesh was termed Sarcode by Dujardin. 

 When recognized in vegetable cells it was called Protoplasm by Mulder, 

 while Remak applied the same name to the substance of animal cells. 

 As the presumed formative matter in animal tissues it was termed Blas- 

 tema, and in the belief that, wherever found, it alone of all substances 

 has to do with generation and nutrition, Beale has named it Germinal 

 matter or Bioplasm. Of these terms the one most in vogue at the pres- 

 ent day, as we have already said, is Protoplasm, and inasmuch as all life, 

 both in the animal and vegetable kingdoms, is associated with proto- 

 plasm, we are justified in describing it, with Huxley, as the " physical 

 basis of life/' or simply " living matter." 



A ceJJ may now be defined as a nucleajd_mass of protoplasm, 1 of 

 microsco-pic size, which possesses sufficient individuality to have a life- 



1 In the human body the cells range from the red blood-cell foiftnr i Q -) to 

 ganglion-cell ( 3 o in -)- 



