480 



SPECIAL PHYSIOLOGY. 



Beroe and other allied forms ; or provided with a lens-like body, embedded in 

 pigment, as in the red or yellow-colored eye-spots found around the borders of 

 the mantle or disc in the Medusa, in immediate contiguity with the so-called 

 auditory sacs, or lithocysts. Nervous filaments probably proceed to them. 



In these cases, and in those Annuloida in which such pigment spots, though 

 destitute of a refractive lens, are situated upon the central nervous ganglion, 

 their sensory office can hardly be denied ; and, as they occupy positions cor- 

 responding to the true ocelli provided with lenses, in other animals, their 

 visual function, however feeble, is placed almost beyond a doubt. 



In the Protozoa, pigment spots are only known to exist in certain Infusoria. 

 As no nervous system has been demonstrated in these low unicellular animal 

 organisms, it has been disputed whether in such creatures, the pigment spots 

 are really visual organs. The undoubted influence of light upon these animals, 

 attracting them, for example, to the light side of the vessel in which they are 

 kept, may be owing to a sensibility inherent either in the sarcodous substance 

 of their bodies, or in nervous granules connected with the pigment spots ; or, 

 as elsewhere remarked, such apparent attraction may be explained by the in- 

 cidental action of the heat associated with the light. 



SPECIAL PHYSIOLOGY. 



THE VEGETATIVE FUNCTIONS. 



THE functions to be considered under this head, are the nutritive 

 and reproductive functions. The former include digestion, absorption, 

 chylification, circulation, nutrition, reparation, sanguification, secre- 

 tion, excretion, and respiration, together with the production of ani- 

 mal heat, muscular force, light, and electricity. 



DIGESTION. 



Amongst other phenomena produced by the waste of the solid con- 

 stituents of the body, and the loss of the fluid, or watery part of the 

 tissues, are the special sensations of hunger and thirst, which have 

 their seat, like other sensations, in the nervous system, and the phe- 

 nomena of which have been already explained (p. 349). These sen- 

 sations of appetite, excite the desire to take food; and by the pro- 

 cess of digestion, the food, thus taken, is prepared for absorption, and 

 conversion into blood. The term food includes all substances, received 

 into the alimentary canal, and used for the support of life, either by 

 supplying the waste constantly occurring in the living animal tissues, 

 or by affording materials for the maintenance of the temperature of the 

 body. Food, therefore, contains substances which have a certain 

 chemical relation to the tissues which it supports. These tissues, be- 

 sides containing water and saline substances, are composed of prox- 

 imate organic principles, having a highly complex chemical constitu- 

 tion (pp. 84 to 86). Food also consists, more or less, of substances 

 having already the same, or a similar chemical composition: for the 

 animal body, so far as is known, has no power of forming such prox- 



