10 INTRODUCTION. 



tions. The medium by which these sensations are conveyed to 

 the brain, the great centre of nervous energy, is through nerves, 

 and the whole apparatus of sensation is termed the nervous sys- 

 tem. In animals not possessed of a brain or spinal column, cords 

 or threads of nervous matter, with thickenings or ganglions at 

 certain distances, form their medium of sensation ; and although 

 in some groups of animals composed of soft parts, or of extreme 

 tenuity, the presence of nerves has not been satisfactorily traced, 

 yet there seems little reason to doubt the existence, in a greater 

 or less degree, of the faculty of sensation in even the lowest of 

 the animal races. 



The material substance of animal bodies, in an anatomical 

 view, may be divided into solids and fluids. The solid portions 

 are named tissues, and are united or combined in various de- 

 grees in the animal organs. These tissues have been dis- 

 tinguished by anatomists by their forms, or by the chemical ele- 

 ments which enter into their composition. They are chiefly the 

 following : 1 . The cellular tissue, forming in the greater num- 

 ber of animals the connecting medium of all their organs, and 

 enveloping and penetrating them by a reticulation of a spongy 

 nature, which takes the form of cells, capable of distension by 

 the fluids which it includes. %. The filwo-gelatmous tissue is 

 a collection of solid, tenacious, and resisting fibres in their longi- 

 tudinal direction, flexible and elastic across, whose use seems to 

 be to communicate movement and resist the efforts of exterior 

 force. It is so named from dissolving in boiling water to the 

 consistence of a jelly. 3. The membranous tissue is a disposi- 

 tion of thin membranous flexible laminae extended like a web, and 

 various in structure and uses. The cutaneous membrane enve- 

 lopes the superficies of the body and permits absorption and ex- 

 halation. It is formed of many layers, and produces the hair, 

 feathers, nails, scales, &c. of the animal body. Other membranes 

 are called mucous or folliculous, because they secrete a viscid 

 fluid, which lubricates their internal surface ; and serous mem- 

 branes are those so named from their internal smooth and polish- 

 ed surface exhaling a very liquid humour. They form thin and 

 transparent sacs without openings, which facilitate the recipro- 

 cal movements of the organs. 4. The vascular tissue is form- 

 ed of continuous membranous branched tubes, to receive, con- 



