THE SENSORI-MOTOR SYSTEM 23 



the pressure of the blood within, cause the vessel to regain its 

 former calibre. 



There are other special mechanisms of this kind; thus the 

 actions of closing and opening the eyes involve the contractions 

 and relaxations of two antagonistic muscles which are present 

 in the eyelids. One of these, the closing set, consists of fibres 

 that form a kind of ring in the upper and lower lids. When 

 these contract, the opening between the latter is diminished, or 

 obliterated. The closing muscle consists of fibres which originate 

 in^the sheath of the optic nerve, and which are inserted into the 

 cartilage of the upper lid ; when they contract the latter is raised, 

 the ring-like closing muscle then becoming relaxed. 



Ca.frilla.ry 

 FIG. 7. MUSCULAR TISSUE. HIGHLY MAGNIFIED. 



The Minute Structure of Muscle. Just a word about this. 

 Examination of a piece of muscle beneath the microscope shows 

 that it is composed of a very great number of minute fibres. 

 Each of the latter is about If inches in length, and it tapers 

 away to a fine point at each end. Numbers of fibres are bound 

 together by connective tissue to form muscle slips, which are 

 similarly joined up to form bundles which constitute the muscle 

 itself. A nerve fibre terminates in each muscle fibre, as shown 

 in Fig. 7. In addition, arteries carry blood to the muscle, where 

 it is distributed through the capillaries and from which it is 

 carried away by the veins. The muscle fibres act at the same 

 time, all of them thickening or contracting when the muscle 

 pulls on its movable bone, or simultaneously lengthening when 

 the muscle relaxes. 



As the figure shows, the fibres are cross-striped, and this stria- 

 tion is, in some way, part of the mechanism of contraction and 

 relaxation. But the muscle fibres that are present in the intestinal 

 or arterial wall are not cross-striped. This presence or absence 

 of striation goes along with a difference in the control over the 

 muscular'actions ; in^most orthe cases^'where the activity of a 

 muscle is controlled by the will (voluntary muscles) the fibres 



