THE SPINAL COED AS A REFLEX CENTRE 333 



animal, and in the female may result in normal parturition which goes on to 

 full term. Pregnancy is accompanied by hypertrophy of the mammary 

 glands and is followed by secretion of milk, so that the young may be suckled 



aoo mm. Hg. 



150 mm. Hg. 



100 mm. Hg 

 b. p. 



Signal 

 Time in 2' 



FIG. 166. Blood pressure tracing from a spinal dog. The signal indicates the 

 time during which the afferent nerve was stimulated. (SHERRINGTON. ) 



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 carry- 

 ing 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 contrac- 

 tion 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 spinal 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. 167). In the intact 

 animal muscle tone is diminished by disease and may be abolished during 

 profound anaesthesia, 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 



