2 PRINCIPLES OF ANIMAL BIOLOGY 



phology. The structure of embryos, or developmental stages, of animals 

 is as much morphology as is the structure of adults. Various names, 

 such as anatomy, histology, cytology, and embryology, have been given 

 to the departments of morphology. They differ from one another in 

 the size and nature or stage of the objects with which they deal, but all 

 are primarily concerned with form and structure. 



Anatomy which is the science of structure was at first concerned with 

 the grosser structures of animals, those that were visible to the unaided 

 eye, such as the divisions of the brain, the spinal cord, and the larger 

 nerves and ganglia, the stomach and intestine and-ihe larger glands 

 and ducts in connection with them, the chambers of the heart and 

 the arteries and veins, the individual bones of the skeleton^ and so 

 on. As aids to dissection gradually came into use, however, more and 

 more minute structures were studied under the name of anatomy, so that 

 today the anatomist considers nothing too minute to fall within his 

 province. In practice, nevertheless, certain phases of morphology are 

 still usually designated by other names. 



Thus histology deals with the grouping of cells of a given kind into 

 masses or layers called tissues. The epidermis of the skin is composed 

 of cells of the same general kind grouped together; it is therefore a tissue. 

 Muscle and bone are tissues, as are also tendons, many glands, and the 

 components of the nervous system. Cyttlogy, contrariwise, is concerned 

 with the minute components of cells, the parts of the nucleus, the minute 

 structure of the living matter, the nature of cell organs, and the like. 

 Embryology has to do with the young or developmental stages of 

 animals. When it is purely descriptive, dealing only with the structures 

 of these young stages, not with the processes going on in them, it is 

 morphological in its nature. 



In creating subdivisions of zoology, the several morphological branches 

 mentioned in the preceding paragraphs have occasionally been elevated 

 to equal rank with the divisions described below; but since they do not 

 differ in principle, all being concerned with structure, it seems better to 

 combine all of them under the single head, morphology. Morphology 

 is historically the oldest of the divisions of zoology, since in early times 

 practically all that was known of animals was their structure. 



Physiology. — The functions of organs, tissues, and cells, the processes 

 carried on in them, are the subject matter of physiology. Not only the 

 more obvious functions, such as digestion, circulation, and respiration, 

 but the obscure chemical reactions involved in growth and repair, belong 

 in this field. The conduction of nerve impulses, the origin of acts of 

 will, the processes of thought, the behavior of animals under given con- 

 ditions, all are the subject matter of physiology. Ordinarily one thinks 

 of those processes which maintain the animal, get and prepare its food, 

 secure a supply of energy, control repair land growth, or eliminate waste 



