GENERAL PROCESSES OF NUTRITION. 427 



The introduction of new matter, so essential to the continuance of the phe- 

 nomena of life, is demanded, on account of the change of the substance of 

 the tissues into what is called effete matter ; and this is discharged from the 

 animal organism, to be appropriated by vegetables and thus maintain the 

 equilibrium between the animal and the vegetable kingdoms. 



It is a well established fact that nearly all of the tissues undergo disassimi- 

 lation, or conversion into effete matter, during their physiological wear in the 

 living organism, while others, like the epidermis and its appendages, are 

 gradually desquamated, and when once formed, do not pass through any 

 farther changes. The whole question of the essence and nature of the nutri- 

 tive property or force resolves itself into vitality. Life is always attended 

 with what are known as the phenomena of nutrition, and nutrition does not 

 exist except in living organisms. At present, physiologists have been able 

 to define life only by a recital of certain of its invariable and characteristic 

 attendant conditions ; and yet there are few if any definitions of life re- 

 garding life as the sum of the phenomena peculiar to living organisms that 

 are not open to grave objections. 



If life be regarded as a principle, it stands in the relation of a cause to 

 the vital phenomena; if it be regarded as the totality of these phenomena, it 

 is an effect. 



In the study of the development of a fecundated ovum, life seems to be 

 a principle, giving the property of appropriating matter from without, until 

 the germ becomes changed, from a globule of microscopic size and compara- 

 tively simple structure, into a complete organism with highly elaborated parts. 

 This organism has a definite form and size, a definite period of existence, and 

 it produces, at a certain time, generative elements, capable of perpetuating 

 its life in new beings. It may be said that an organism dies physiologically 

 because the vital principle, if such a principle be admitted, has a limited 

 term of existence ; but on the other hand, the fully developed living organ- 

 ism, called an animal, presents many distinct parts, each endowed with 

 an independent property called vital, that property recognized by Haller 

 in various tissues, under the name of irritability ; and it is the co-ordinated 

 association of these vitalities that constitutes the perfect being. These are 

 more or less distinct ; and a sudden and simultaneous arrest of the physio- 

 logical properties in all the tissues, in what is called death, is not often ob- 

 served. For example, the nerves may die before the muscles, or the muscles, 

 before the nerves. It is found, also, that physiological properties, apparently 

 lost or destroyed, may be made to return ; as in resuscitation after asphyxia 

 or in the restoration of muscular or nervous excitability by injection of blood. 



The life of a fecundated ovum is the property which enables it to undergo 

 development when placed under favorable conditions ; and by the surround- 

 ing conditions, its development may be arrested, suspended or modified. 

 The life of a non-fecundated ovum is like that of any ordinary anatomical 

 element. 



The life of an anatomical element or tissue in process of development is 

 the property by virtue of which it arrives at its perfection of organization 



