THE LOWER LIMB 



591 



The Interphalangeal Joints. 



These joints belong to the class diarthrosis, and to the subdivision 

 ginglymus. They are formed by the distal end of one phalanx 

 and the proximal end of the adjoining phalanx. The hgaments are 

 three in number, two lateral and an inferior or plantar fibrous plate, 

 the extensor tendon taking the place of a dorsal ligament. Each 

 joint is provided with a synovial membrane. The ligaments, 



Fig. 273. — Sagittal Section of Ankle and Foot, passing through 

 THE Great Toe. 



I, Spring Ligament ; 2, Inner Sesamoid Bone. 



synovial membranes, and movements of these joints closely corre- 

 spond with those of the interphalangeal joints of the hand. 



The only joint in the lower limb which has interartieolar fibro-eartilages 

 is the knee-joint. The hip-joint has a circumferential fibro-cartilage, namely, 

 the cotyloid ligament. 



Development of the Limbs. 



In the third week of embr>'onic life two longitudinal ridges make their 

 appearance, one on either side, external to the mesodermic somites. The 

 rudiments of the fore- and hind-limbs (exclusive of the i>ectoral and pelvic 

 girdles) appear as processes of these ridges. These prominences cissume the 

 form of distinct outgrowths, which are called the limb-buds. The limb-buds 

 undergo elongation, and each bud becomes divided into two segments by 

 means of a transverse groove. The distal segment represents the nianus or 

 pes, as the case may be. The proximal segment soon becomes differentiated by 

 means of another groove into two parts — namely, antibrachium and brachiutn 

 for the fore-limb, and leg and thigh for the hind-hmb. 



Stractare of a Limb-Bud. — Each limb-bud consists of the following ele- 

 ments: (1) A core of mesenchyme (covered with ectoderm), derived from 

 several of the contiguous somites; (2) muscular tissue, derived from the 

 muscle-plates of several contiguous somites; and (3) prolongations (anterior 

 primary- divisions) from the contiguous spinal nerve-segmentsl 



The bones are developed from the core of mesenchyme, which first undergoes 

 chondrification and subsequently os.sifi cation. The mesenchyme in the 

 intervals between the osseous frsuneworks assumes a hollow condition, and 



