THE SPINAL CORD AS A REFLEX CENTRE 



tracts forcibly, the contraction being accompanied by relaxation of the 

 sphincter and followed by rhythmic contractions of the urethral muscles ; 

 accumulation of faeces in the rectum leads to their normal evacuation. With 

 a little assistance impregnation may be effected in or by such a maimed 



200 mm. Hg. 



150 mm. Hg. 



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

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



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 sub- 

 ject. 



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 mav_be_abolished during profound 

 ;>iiastli<'sia. as it is indeed in the condition of shock. 



Much light has been thrown on the factors which determine muse 

 tone by a study of the ' tendon phenomena ' of which the knee-jerk is the 



