474 OF NUTRITION. 



and also, but to a less degree, with the organs of digestion. But in some situations, both 

 in man and in the inferior creatures, it is difficult to determine what functions the ciliary 

 motion can perform. Such are, in man, the ventricles of the brain ; and, in the frog, the 

 closed cavities of the pericardium and peritoneum. Here there are no excretory orifices, 

 towards which the current might set. 



What is the cause of ciliary motion? We have shown it to be independent of the 

 blood and of the nerves, arid to resist those depressing causes which usually put a stop 

 to the action of contractile tissue. It requires for its continuance three conditions: a 

 perfect epithelium cell ; moisture, not of too great density ; and a temperature within cer- 

 tain limits. From Schwann's observations it appears that cells exhibit a power of endos- 

 mose; that a chemical change occurs in the fluids in contact with them; and that a 

 movement of their internal granules may be seen under certain circumstances. If 

 ciliated epithelium cells exert an attraction of endosmose upon the surrounding fluid, 

 may not this physical phenomenon afford a clue to determine the cause of the movement ] 



A very remarkable movement is manifested by certain particles found in the secretion 

 of the testicle, which prevails most extensively throughout the animal series, and is even 

 found among plants. From the regularity of these movements and their resemblance to 

 those of minute animals, a place has been assigned by naturalists to the particles in ques- 

 tion, in their zoological classifications, under the name " Cercarias Seminis" Spermatozoa, 

 or Spermatic animalcules, and Ehrenberg refers them to the Haustellate Entozoa ( 734). 

 These particles consist chiefly of a long filament or tail, which is sometimes swollen at 

 one extremity, to form the body of the supposed animalcule. The motions consist in a 

 sculling action of the tail, or a slight lateral vibration of it. In many of its conditions it 

 closely resembles ciliary motion; and its duration after death, or after the separation of 

 the fluid, is pretty much the same as that of the ciliary movements. The particles are 

 extremely minute, even measured in their length; but especially so in thickness. They 

 are, therefore, well adapted to obey those impulses which we have shown to be capable 

 of giving rise to molecular motions.* M. C.] 



622. It is interesting to observe that the Epidermoid tissues have the simplest 

 structure of any solid parts in the whole animal body ; and that they are the 

 most readily renewed. There appears no limit to their power of reproduction ; 

 but, when once formed, they undergo no transformation. They are not repro- 

 duced from each other ; and their origin appears to be in germs supplied by the 

 basement membranes, through which the plasma transudes, at whose expense 

 they are developed. The duration of their lives seems to vary greatly in dif- 

 ferent parts. The Epidermic cells, being exposed on one side to the air, soon 

 desiccate, and become converted into flattened scales ; but by the adhesion of 

 these to each other, forming the outer layers of the cuticle, they are not rapidly 

 abraded, except where there is much friction, or any other cause producing an 

 unusual desquamation. The Epithelium covering the internal Serous surfaces, 

 and some of the Mucous surfaces, appears to be still more durable, and less 

 frequently needing renewal. But on most of the Mucous surfaces, the loss 

 and renewal of Epithelium are continually taking place. It will be shown 

 hereafter ( 651), that the Epithelium-cells are the real agents in the elabo- 

 ration of the secretions furnished by these membranes and their prolongations ; 

 whether they consist of simple mucus, or are of an altogether different cha- 

 racter ; and the production and casting off of these cells will take place with 

 a rapidity proportional to the activity of the secreting processes. 



623. The Hair takes its origin in little follicles or open sacs, formed by the 

 inversion of the Cutis, and lined by a reflexion of the Epidermis. Each of 

 these follicles contains what is designated as the bulb of the hair ; which has 

 generally been described as the soft newly-formed part, that subsequently con- 

 solidates into the outer portion ; whilst the still softer interior of the bulb, 

 termed the pulp, has been supposed to furnish the matter of the interior. 

 The follicle is extremely vascular ; and even the bulb is reddened by a minute 

 injection, though no distinct vessels can be traced into it. It has, until recently, 

 been imagined that the Hair, like the other extra-vascular tissues, is a mere 

 product of secretion; its material (horny matter, 620) being elaborated from 



[* Todd and Bowman on the Physiological Anatomy and Physiology of Man, p. 61.] 



