58 ESSENTIALS OF PHYSIOLOGY. 



forming a cell station in the grey matter, and being conveyed by other 

 neurons to the anterior column of the opposite side. They also ulti- 

 mately reach the optic thalamus, and pass to the cerebral cortex by a 

 fresh relay (fig. 28). 



SECTION IV. 

 REFLEX ACTION. 



In the course of evolution survival has depended, to a large extent, 

 upon the rapidity and efficiency with which reflex movements are 

 carried out; and the essential characters of a reflex action are (1) that 

 it should be rapid, and (2) that it should be co-ordinate, that is, that 

 the muscles concerned in the reflex act should contract together so 

 as best to attain the end for which the reflex exists. Further, the 

 response to a stimulus must be limited in its extent, and must not 

 involve the whole muscular system. Finally, evolution is made 

 possible by the capacity of the central nervous system to form new 

 reflexes, and this capacity is the basis of habit and of educability. It is 

 in this respect that the nervous system of man has become so much 

 more highly differentiated and complex than that of the lower animals. 



In man the reflex functions of the spinal cord have become to a 

 large extent subordinate to those of the brain ; and the spinal cord, 

 when separated from the brain as a result of injury, displays a compara- 

 tively feeble reflex power. In the lower animals the spinal reflexes are 

 more pronounced, and can be readily studied either in the pithed frog 

 or in a spinal mammal, i.e. an animal which is allowed to recover after 

 transection of the spinal cord, this being usually made in the upper 

 thoracic region. 



The Reflex Arc. When a stimulus falls upon a sensory surface or 

 sense organ, which is called a receptor, it gives rise to an impulse which 

 is conveyed by an afferent nerve to the spinal cord. Here it travels 

 through a number of neurons, where its character is modified, and 

 finally the impulse leaves the spinal cord along an efferent nerve to 

 reach the muscle or gland which responds to the stimulus. This path 

 is called a reflex arc, and consists of (1) a receptor, (2) afferent nerve, 

 (3) neurons in the spinal cord, (4) efferent nerve, and (5) muscle or 

 gland (fig. 12). 



Interruption of the arc at any point abolishes the reflex action. 



The time occupied by an impulse in travelling from the receptor 

 through the central nervous system to the muscle or gland (effector 

 organ) is called the total reflex time. A part of this time is occupied 

 in the transmission of the impulse along the afferent and efferent 



