362 ANATOMICAL TBGHNOLOGT. 



§ 974. AA. lurabales (6 pairs). — The lumbar arteries arise from the dorsal side of the 

 aorta opposite the intervertebral fibro-cartilagea. They extend directly dorsad for a short 

 distance, and then turn laterad to be distributed to the ental or attached surface of the 

 muscles on the ventral and lateral aspects of the spinal column. A small branch usually 

 enters the intervertebral foramen to supply the myelon. 



§ 975. Demonstration of the AA. lumhales. — Turn the stomach 

 to the right, cut the left renal vessels and remove the left kidney 

 and the left half of the diaphragm. Draw the aorta dextrad. 

 Commence opposite the last rib, and with the forceps and tracer 

 clear away the fat and connective tissue on the dorsal and sinistral 

 aspects of the aorta. From the great caudal projection of the dor- 

 sal part of the diaphragm, the 1st, 2d, and sometimes 3d, pairs of 

 AA. lumbales are given off within the thorax ; hence the necessity 

 of removing the diaphragm. The last pair is given oflf by the aorta 

 just caudad of the origin of the A. iliaca externa. Tear and cut 

 away carefully the muscles and connective tissue, and trace the 2d 

 or 3d A. lumbalis and its distribution as described above (§ 974). 



STRUCTURE OF THE HEART AND BLOOD VESSELS. 



§ 976. Obvious Structure of the Heart. — (A) The heart is covered by a serous mem- 

 Irane — the ental (inner) layer of the pericardium (§ 910). 



(B) Its main substance is composed of muscle, apparently but not really Uke the ordi- 

 nary voluntary muscles. Compare g§ 704 and 705. 



The arrangement of muscular fasciculi in the auricles seems to be comparatively simple, 

 namely, in two layers, of which the octal have a circular and the ental a longitudinal 

 direction (Strieker, A, 183). But in the ventricular walls the arrangement is very complex 

 (Quain, A, I, Fig. 179). According to Pettigrew (A, 194), there are no less than seven 

 more or less distinct layers, the fibers of which vary in direction from nearly circular to 

 nearly longitudinal, vrith several degrees of obliquity. The author just named recom- 

 mends (p. 193, note 2) that for the study of the layers, the cavities should be filled with 

 bran and the organ then boiled for from 4-10 hours, according to size, and kept in alcohol 

 for a fortnight before dissecting. 



(C) The cavities (Fig. 93) are lined by a firm serous membrane — the endocardium. 



(D) Vahes and chord(B tendineoe (§§ 864, 881-884). 



§ 977. Microscopic Structure. — (A) The serous covering and lining of the heart has 

 the structure belonging to serous membranes generally (§ 730). 



(B) " The muscular fibers of the heart differ remarkably from those of involuntary 

 muscular organs in general, inasmuch as they present transverse stria. The striae, how- 

 ever, are less strongly marked and less regular, and the fibers are smaller in diameter than 

 in the voluntary muscles (§ 705). They differ also from these in being made up of quad- 

 rangular cells joined end to end, and often presenting a branched or forked appearance 

 near one extremity. Each cell has commonly a single clear oval nucleus situated near 

 the center ; occasionally two nuclei are seen. The cell substance is faintly striated lon- 

 gitudinally as well as transversely ; it presents no indication of an investing membrane or 

 sarcolemma. As stated by Strieker, however (A, 182) the ' cells of muscles, like all other 



