THE SPINAL CORD AS A REFLEX CENTRE 



377 



may be effected in or by such a maimed animal, and in the female 

 may terminate at full term in normal parturition. Pregnancy is 

 accompanied by hypertrophy of the mammary glands and is followed 

 by secretion of milk, so that the young may be suckled as in a normal 

 animal. Similar phenomena have been observed in the human 

 subject. 



Such an animal furnishes us with an opportunity of analysing 

 the factors which are involved in the maintenance 

 of muscle tone, as well as in the carrying out of 

 the simplest reflexes involving contractions of the 

 skeletal muscles. 



MUSCULAR TONE 



Every muscle in the body is in a condition of 

 slightly continued contraction which keeps it tense, 

 so that when it contracts in response to a stimulus 

 there is, so to speak, no ' slack ' to be taken up 

 before the muscle begins to pull on its attachments. 

 This tone is seen in the retraction undergone by 

 muscles or tendons when they are divided in the 

 living animal. 



If a frog possessing only spinal cord be hung 

 up by its jaw, the limbs will be observed to occupy 

 a position which is short of complete extension. 

 The tone of the muscles which is concerned in the 

 maintenance of this attitude is at once abolished 

 by the destruction of the cord. It may be abolished 

 on one side by section either of the anterior roots 

 going to the muscles, or of the posterior roots 



coming from the muscles (Fig. 168). In the intact 



f i j- i_ j i_ j- j FIG. 168. Hind part 



animal muscle tone is diminished by disease and O f a S pi na i fr g, 



may be abolished by any condition of profound 

 anesthesia, as it is indeed in the condition of shock. 

 Much light has been thrown on the factors 

 which determine muscular tone by a study 

 of the * tendon phenomena ' of which the 

 knee-jerk is the most familiar example. If the leg is allowed 

 to hang loosely in a position of slight flexion at hip and knee and 

 the patellar tendon be struck, the extensor muscles of the thigh 

 contract and raise the leg. This phenomenon is known as the knee- 

 jerk. Similar ' tendon reflexes ' can be obtained in other muscles, 

 such as the tendo Achillis, the triceps, and the extensor muscles 

 of the wrist, but with not so great ease as is the case with the knee. 

 The knee-jerk is not altered by rendering the tendon anaesthetic by 



hung up by the 

 jaw. The posterior 

 roots of the nerves 

 to the left hind 

 limb have been 

 divided. 



(BECHTEEEW.) 



