288 SCIENCE OF AGRICULTURE. Part II. 



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 net-work, between the different parts of which an intimate connection sub- 

 sists. In consequence of this arrangement, it is often matter of very great difficulty 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. 



1877. The brain in the animals without vertehree, 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. Independent of very remarkable differences 

 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 belonging to it are 

 arranged. In the first, the brain is situated upon the oesophagus. It presents different 

 forms, according to the species. It appears more like a ganglion, than the brain of the 

 vertebral animals. It sends off several nerves to the mouth, eyes, and feelers. One on 

 each side passes round the oesophagus ; these 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 sys- 

 tem prevails, belonged to the great division termed Mollusca. In the second, the brain 

 is situated as in the mollusca, sends 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 

 tlie body consists. 



1878. The functions of the brain and nervous si/stem ; 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 Composition of the Bodies of 



Animals. 



1879.' The elementary principles of the animal kingdom have been ascertained with 

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

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

 brought into union in the animal system by the agency of the vital principle, their state 

 of combination may be expected to differ widely from the ordinary results of electric 

 attraction. Wlien such compounds of organization are submitted to analysis, the influ- 

 ence of the vital principle having ceased, the products obtained may be regarded, in 

 many cases, as modifications of the elements of the substance, occasioned by the pro- 

 cesses employed, rather than the display of the number or nature of the ingredients, as 

 they existed previous to the analytical operations. Hence the great caution requisite in 

 drawing conclusions regarding the composition of animal bodies. 



1880. The elementary substances which are considered as entering into the parts of ani- 

 mals, are carbon, hydrogen, oxygen, azote, phosphorus, sulphur, fluoric acid, muriatic 

 acid, iodine, potash, soda, ammonia, lime, magnesia, silica, iron, and manganese. 



1881. Carbon exists in various states of combination in the fluids as well as the solids of every animal, 

 and has been detected in the form of charcoal in the lungs. When animal substances are exposed to a 

 high temperature in closed vessels, the charcoal which is produced differs considerably from that which is 

 obtained by the same means from vegetables. It is more glossy in appearance, and is incinerated with 

 much greater difficulty. 



1882. Hydrogen is universally distributed in the animal kingdom ; it occurs as a constituent ingredient 

 of all the fluids, and of many of the solids. It is invariably in a state of combination with charcoal; for, 

 as far as we know, it has never been detected in an uncombined or separate state. It has been found in 

 the human intestines, in the form of carburetted hydrogen. 



1883. Oxygen is equally widely distributed with the preceding, in the fluids and solids of all animals, A 

 constant supply of it from the atmosphere is indispensably necessary to the continuation of animal life. 

 It occurs, not only in combination with other bodies, biit probably, likewise in a separate state, in the 

 air-bag of fishes, in which it is found, varying in quantity, according to the species, and the depth at 

 which the fishes have been caught. It is common, in union with charcoal, forming carbonic acid. 



1884. Azotic gas is very widely distributed as a component part of animal substances. It occurs in almos 

 all the fluids, and in those solid parts which have carbon as'a base. The almost universal prevalence of thi 

 principle in animal substances, constitutes one of the most certain marks by which they may be distin 



