PASSIVE ORGANS OF LOCOMOTION. 543 



a concentrated effort of the will, the proper muscles are then brought into action. 

 This remarkable action of the muscles of the thorax and abdomen, due to simple 

 effort and independent of the particular muscular act that is to be accomplished, com- 

 presses the contents of the rectum and bladder and obstructs very materially the venous 

 circulation in the large vessels. It is well known that hernia is frequently produced in 

 this way ; the veins of the face and neck become turgid ; the conjunctiva may become 

 ecchymosed; and sometimes aneurismal sacs are ruptured. An effort of this kind is 

 generally of short duration, and it cannot, indeed, be prolonged beyond the time during 

 which respiration can be conveniently arrested. At its conclusion there is commonly a 

 prolonged expiration, which is audible and somewhat violent at its commencement. 



There are degrees of effort which are not attended with this powerful action of the 

 muscles of the chest and abdomen, and in which the glottis is not completely closed ; 

 and an opening into the trachea or larynx, rendering immobility of the thorax impossi- 

 ble, does not interfere with certain acts that require considerable muscular power. If 

 we examine a dog with the glottis exposed, when he makes violent efforts to escape, we 

 can see that the opening is firmly closed. This fact we have often observed in vivisec- 

 tions ; but Longet has shown that dogs with an opening into the trachea are frequently 

 able to run and leap with " astonishing agility." He also saw a horse, with a large 

 canula in the trachea, that performed severe labor and drew heavily -loaded wagons in 

 the streets of Paris. 



Passive Organs of Locomotion. 



It would be out of place to describe fully and in detail all of the varied and complex 

 movements produced by muscular action. Many of these, such as the movements of 

 deglutition and of respiration, are necessarily considered in connection with the func- 

 tions of which they form a part ; but others are purely anatomical questions. Associ- 

 ated and antagonistic movements, automatic and reflex movements, etc., belong to the 

 history of the motor nerves and will be fully considered under the head of the nervous 

 system. 



The study of locomotion involves a knowledge of the physiological anatomy of cer- 

 tain passive organs, the bones, cartilages, and ligaments. Although a complete history 

 of the structure of these parts trenches somewhat upon the domain of anatomy, we are 

 tempted to give a brief description of their histology, as it will complete our account of 

 the tissues of the body, with the exception of the nervous system and the organs of 

 generation, which will be taken up hereafter. 



Locomotion is effected by the muscles acting upon certain passive, movable parts. 

 These are the bones, cartilages, ligaments, aponeuroses, and tendons. We have already 

 described the fibrous structures, and it only remains for us to study the bones and car- 

 tilages. 



Physiological Anatomy of the Bones. The number, classification, and relations of the 

 bones are questions belonging to descriptive anatomy ; and the only points we propose to 

 consider refer to their general, or microscopical structure. 



Every bone, be it long or short, is composed of what is called the fundamental sub- 

 stance, marked by microscopic cavities and canals of peculiar form. The cavities con- 

 tain corpuscular bodies, called bone-corpuscles. The canals of larger size serve for the 

 passage of blood-vessels, while the smaller canals (canaliculi) connect the cavities with 

 each other and finally with the vascular tubes. Many of the bones present a medullary 

 cavity, filled with a peculiar structure, called marrow. \In almost all bones there are two 

 distinct portions ; one, which is exceedingly compact, and the other, more or less spongy 

 or cancellated. The bones are also invested with a membrane, containing vessels and 

 nerves, called the periosteum. 



The method usually employed in the study of the bones is by thin sections made in 



