THORACIC DUCTS. 363 



naked cells, must present a peripheral investment ; ' that Is, like all cells ■wliicli form 

 part of a complex, compact structure, and do not simply float free in a liquid as do the 

 leucocytes in the blood. The muscular fibers of the heart freely divide and anastomose, 

 the junction with neighboring fibers being effected by the medium of the cell-ofFsets 

 above noticed." Strieker, A, 179 ; Quain, A, II, 119, 240, 261. 



§ 978. Arteriae. — Their obcious structure is that of smooth-bored, thick-walled tubes 

 which retain their form when cut. They are elastic and flexible. 



§ 979. Microscopic Structure. — They are composed of three well-defined coats ; — (A) 

 Ectal or outside coat, tunica adtentitia, of rather loose elastic and white connective tissue, 

 with a general longitudinal direction. (B) Intermediate or middle coat, composed of elas- 

 tic and white connective tissue and circularly arranged muscular fibers. (C) Ental or 

 Inner coat, composed of elastic tissue covered by endothelium on its free surface. 



§ 980. Muscular and Elastic Types of Arteries. — Ranvler and others have divided the 

 arteries into the two classes just named, from the preponderance of elastic tissue or of mus- 

 cular fibers in the middle coat. 'I'he larger arteries, like the carotids, the axillary and the 

 aorta, contain very little muscular tissue, and hence they belong to the elastic type. The 

 arteries of the limbs and the smaller arteries generally contain a large amount of muscu- 

 lar tissue in their middle coat, and hence represent the muscular type. 



§ 981. Venae. — The obvious structure of tliese is the same as that of the arteries, but 

 the walls are thin, so that they collapse when cut. 



§ 982. Microscopic Structure. — There are three coats as with the arteries. The white 

 connective tissue is more abundant in the middle coat. Smooth muscle is often present in 

 the ectal coat of some large veins, like the Vena cava, while it is entirely wanting in 

 others, as in most of those of the brain and pia mater ; Quain, A, II, 173. 



§ 983. Capillaries. — " The wall of the capillaries proper is formed entirely of a simple 

 epithelioid layer, composed of flattened lanceolate cells joined edge to edge, and continuous 

 with the corresponding layer which lines the arteries and veins." Quain, A, 11, 177. 



§ 984. The structure of the lymphatic vessels very closely resembles that of the blood 

 vessels. Quain, A, II, 186. 



THORACIC DUCTS AND LYMPHATIC VESSELS. 



References. — Quain, A, I, 504 ; Gray, A, 589 ; Cuvier, A, VI, 60 ; Qegenbaur (Lan- 

 kester). A, 599 ; Hyrtl, A, 750 ; Bernard, A, 253 ; Leyh, A, 656 ; Owen, A, III, 511 ; 

 Milne- Edwards, A, iV, 503; Chauveau, A, 675; Chauveau (Fleming), A, 634; Gurlt, 

 A, 681. 



Instruments and materials the same as for the blood vessels 

 (§912). 



§ 985. Specimen, Preparation, Posture and Dissection. — Employ a young, but 

 especially a lean cat. Feed the cat with milk or fat meat, and after two hours kill with 

 chloroform. Inject the femoral artery with red starch (Fig. 39, § 352 and leaflet between 

 pp. 140, 141), or open the abdomen as for abdominal viscera (§ 710) and inject the aorta just 

 caudad of the origin of the inferior mesenteric artery (Fig. 101). After the arterial injec- 

 tion is made and the abdominal cavity opened, draw the intestines to the right and a large 

 lacteal vessel will be seen passing from the large mesenteric glands near the csecum, dorso- 

 cephalad toward the cephalic end of the left kidney. Where this lacteal vessel crosses 

 the superior mesenteric vein (Fig. 103), or slightly centrad of that point, remove one wall 



