INTRODUCTION. XVil 
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; aad 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 nervors 
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 aninal 
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 
laminz, 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 tissue is formed cf con- 
3 ’ 
