Book II. ANIMAL CHEMISTRY. 289 



stable he stands. Dogs and cats form their bodies into a circle, while birds place theit 

 heads under their wings. 



1911. The ordinary mode of sleep is likewise exceedingly various in different animals, and in the same 

 animal is greatly influenced hy habit. It in general depends on circumstances connected with food. It is 

 probable, that ail animals, however low in the scale, have their stated intervals of repose, although we are 

 as yet unacquainted either with the position which many of them assume, or the periods during which 

 they repose. 



SuBSECT. 3. Structure of the Nervous System. 



1912. The nervous system, by containing the organs of sensation and volition, is that 

 which distinguishes animal from vegetable beings. It consists, in the vertebrated animals, 

 of the brain, the spinal marrow, and the nerves. 



1913. The brain, exclusive of its integuments, appears in the form of a soft, compres- 

 sible, slightly viscous mass. The spinal marrow originates with the brain, and consists of 

 four cords united in one body. The nerves, also, originate in the brain or spinal mar- 

 row. Some of them appear to have a simple origin ; but, in general, several filaments, 

 from different parts of the brain or spinal marrow, unite to form the trunk of a nerve. 

 This trunk again subdivides in various ways; but the ramifications do not always ex- 

 hibit a proportional decrease of size. It frequently happens that the branches of the 

 same nerve, or of different ones, unite and separate repeatedly within a small space, 

 forming a kind of network, to which the name plexus has been applied. Sometimes 

 filaments pass from one nerve to another ; and, at the junction, there is usually an 

 enlargement of medullary matter termed a ganglion. Numerous filaments, from dif- 

 ferent nerves, often unite to form a ganglion, from which proceed trunks frequently of 

 greater magnitude than the filaments which entered. Thus nerves, very different in their 

 origin, form communications with one another ; so that the whole nervous system may 

 be considered as a kind of network, between the different parts of which an intimate con- 

 nection subsists. In consequence of this arrangement, it is often matter of very great 

 diflSculty to ascertain the origin of those filaments, which unite to constitute the trunk of 

 a nerve. In some instances, they appear to arise from the surface of the brain or spinal 

 marrow ; in other cases, from the more central parts. 



1914. The brain, in the animals without vertebrce, is destitute of the protecting bony 

 covering, which forms the head and back bone in the vertebral animals. The brain 

 itself is much more simple in its structure. Independently of very remarkable dif- 

 ferences in the structure of the nervous system in the different genera of invertebral 

 animals, there may still be perceived two models, according to which, the organs belong- 

 ing to it are arranged. In the first, the brain is situated upon the oesophagus, and presents 

 different forms according to the species, appearing more like a ganglion than like the brain 

 of the vertebral animals. It sends off several nerves to the mouth, eyes, and feelers. Two, 

 one on each side, pass round the oesophagus, and, uniting below, form a ganglion in some 

 cases larger than what is considered the true brain. From this ganglion, nerves are 

 likewise sent off to different parts of the body. The animals in which this nervous system 

 prevails belong to the great division termed Mollusca. In the second, the brain is situ- 

 ated as in the Mollusca, sending out nerves to the surrounding parts, and likewise one 

 nerve on each side, which, by their union, form a ganglion, from which other nerves 

 issue. This ganglion produces likewise a nervous cord, which proceeds towards the 

 extremity of the body, forming throughout its length ganglia, from which small nerves 

 proceed ; this cord, at its commencement, is, in some cases, double for a short distance. 

 It has been compared to the medulla oblongata, and spinal marrow of the vertebral 

 animals. This kind of nervous system is peculiar to the annulose animals. There are 

 usually ganglia on the nervous cord, corresponding with the number of rings of which 

 the body consists. 



1915. The functions of the brain and nervous system ; the organs of perception, as of 

 touch, of heat, of light, of hearing, of smell, and of taste ; and also the faculties of the 

 mind, we pass over as belonging chiefly to the anatomy and physiology of the human 

 frame, and therefore less immediately connected with the animals used in agriculture. 

 The reader will find these subjects ably treated by Dr. Fleming. 



Chap. III. 



Animal Chemistry ; or the Substances which enter into the Compodlion of the Bodies of 



Animals. 



1916. The elementary principles of the animal kingdom have been ascertained with 

 considerable precision ; but the binary, ternary, or other compounds which they form, 

 have not been investigated with so much success. As these various ingredients are 



U 



