FUM>AM1'..\TAL PRINCIPLES OF ANATOMY. 17 



For the sake of gaining a conception of the architecture of sheaths and the 

 rationale of their formation by displacement of connective tissue, let the follow- 

 ing explanation be studied : Figure 1 represents a pot of earth in which various 

 seeds have been planted ; the pot is covered with a layer of absorbent 

 cotton, and exposed to the sun and rain. In time the cotyledons begin to 

 peep through the cotton ; as each becomes larger in diameter, it displaces 

 more cotton. As the cotton displaced by the growing plant may represent the 

 sheath of the plant, so, in like manner, to aid the memory, let us compare a 

 growing artery, bone, muscle, nerve, or gland surrounded by connective tissue 

 (which represents the cotton in the simile of the growing plant) to the plant. 

 The connective tissue would be displaced as the anatomical structure became 

 thicker, and this displaced tissue we call the sheath, capsule, tunic, toga, or 

 peri structure. 



Now, by the same process account for periosteum, perimysium, perineurium, 

 and all peristructures and capsules. 



PERIOSTEUM AND ITS FUNCTIONS. Periosteum is the anatomical investment 

 of bone. It covers all the bone except the articular surfaces. It is, as you 

 must demonstrate on your dissection, most intimately adherent to those parts of 

 the bone having the greatest number of irregularities. Its functions are as 

 follows : 



1 . Osteogcnetic, for it makes bone grow in thickness. 



2. Protective, since it hinders progress of contiguous inflammation. 



3. Attac/unental, since it gives attachment to muscles. 



4. Nutritive, since it feeds the bone with blood. 



5. Ligamentous, since it forms all capsular ligaments. 



6. Retentive, since it tends to retain ends of broken bones. 



EMINENCES, DEPRESSIONS, AND SURFACES OF BONE are determined in shape 

 by the muscles associated therewith, and take the name of the muscle. You are 

 to look on muscular traction as the factor that determines the size of an eminence 

 or the depth of a fossa. In fact, the surfaces of bone are named according to 

 the occupant. In a given bone, as the humerus, name the articular surfaces 

 according to the occupants: (i) Scapular; (2) radial; (3) ulnar. Name the 

 surfaces according to the muscles. The posterior surface has an upper and a 

 lower tricipital surface, corresponding to the humeral heads of the triceps, etc. 

 Bicipital tuberosity, iliac fossa, gluteal ridge, deltoid impression, are instances. 

 You will observe that this rule is not always observed ; still the value of the rule 

 is no less. 



ANATOMICAL WEAK POINTS comply with two conditions: (i) A location in 

 the continuity of the structure where there is a sudden abrupt change in the 

 direction. (2) A functional area, as between skin and mucous membrane, as 

 where the small intestine opens into the large. In these localities fractures occur 

 in bone, aneurisms in arteries, morbid growths at muco-cutaneous areas. In 

 your dissection bear these areas in mind. Look for intussusceptions at the 

 weak point's in the colon ; for fractures at the necks of bones, etc. 



ANATOMICAL ROOTS are places uncovered by serous membrane, in connection 

 with viscera, where the vessels, nerves, and conduits enter to carry on (i) the 

 functional activity and (2) the nutritive activity of the organ. In your dissection 

 you will give special study to the root-structures of the heart, lung, liver, spleen, 

 kidney, intestine, ovary, testicle, etc. The functional activity is represented by 



