INTRODUCTION. XV11 



of respiration. The spine is generally prolonged behind, and forms 

 the tail in many animals. 



The mechanical apparatus by which animals acquire the know- 

 ledge of what is around them, are termed organs of sense ; and the 

 impressions made on these by external objects, sensations. 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 system. 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, com- 

 posed 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 degrees, in the animal 

 organs. These tissues have been distinguished by anatomists by 

 their forms, or by the chemical elements which enter into their compo- 

 sition. They are chiefly the following : 1. The cellular tissue, form- 

 ing in the greater number 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 disten- 

 sion by the fluids which it includes. 2. The fibro- gelatinous tissue is 

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

 nal direction, flexible and elastic across, whose use seems to be to com- 

 municate 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 disposition of thin, membranous, flexible 

 lamina?, extended like a web, and various in structure and uses. The 

 cutaneous membrane envelopes the superficies of the body, and per- 

 mits absorption and exhalation. 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 membranes are those so named from their internal smooth and 

 polished surface, exhaling a very liquid humor. They form thin and 

 transparent sacs, without openings, which facilitate the reciprocal 

 movements of the organs. 4. The vascular tissice is formed of con- 



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