THE NERVOUS SYSTEM 389 



one of the motor cells, and this immediately liberates an impulse, which, 

 emerging by the inferior root, travels down the motor nerve to the 

 muscle. As many sensory nerves are always stimulated, many motor 

 nerves are called into action, and these are so connected and associated 

 together as to produce purposive movements. 



In the illustration just given, the reflex movements are said to be 

 without consciousness, and they are analogous to those that are constantly 

 taking place in the uninjured animal in the movements of the intestine, of 

 the heart and blood-vessels, of respiration, and of the ducts of glands; but in 

 a large number of cases reflex acts are accompanied by consciousness, as in 

 the case of winking when the eyelashes are touched or the eye is exposed 

 to a bright light, or of coughing or vomiting from tickling the throat with 

 a feather, or of micturition from over-distension of the bladder. When the 

 animal desires to perform one of these purposive and complex movements, 

 it does not transmit a separate impulse to the several muscles implicated 

 in the act, for it knows nothing of them, but by an act of the mind it 

 transmits a mandate to a group of cells which have learned to act together 

 in a definite order and to produce the required effect. Such centres are 

 named co-ordinating centres or nuclei, and of these there are many, as, for 

 example, those governing the movements of the rectum and bladder in the 

 acts of discharging the faeces and urine; those required for parturition, 

 and for the erection of the penis and the ejaculation of semen, which are 

 chiefly situated in the lumbar and sacral regions of the cord; and finally, 

 the contraction of the blood-vessels of the abdomen and lower limbs, 

 which are chiefly situated in the dorsal region. 



There appears also to be present in the cord, centres that control the 

 production of animal heat and of the secretion of sweat, these eftects being 

 in part due in both instances to changes in the size of the blood-vessels, 

 and in the case of the sweat secretion to direct action of the nerves on the 

 sweat glands. 



We must regard the cord as endowed to an eminent degree with iux- 

 pressionability, and the power of inducing reflex acts without consciousness. 

 But the cord is not a receiver of nervous impressions and a generator of 

 nerve impulses only; it is also a conductor transmitting impressions made 

 upon the skin to the medulla oblongata, cerebellum, and cerebrum on the 

 one hand, and impulses originating in these parts to the muscles ot the 

 limbs and trunk, and to the other organs of the body. 



The medulla oblongata, while it is a prolongation upwards of the spinal 

 cord, and transmits impressions both from the cord to the brain and cere- 

 bellum, and downwards from these parts to the cord, is also itself a very 

 important centre, containing many groups of cells which preside over and 



