18 Practicum III. Opening the Thorax. 



be opened prematurely. Save the larger VESSELS and NERVES so far as 

 practicable. The JUGULAR VEIN has already been seen in PI. Ill ; it 

 may commonly be recognized from its location, its size, and from con- 

 taining blood. 



$ 7. The Thoracic Framework. Manipulate the thorax so as to rec- 

 ognize by the touch the constituents of its framework. The SPINE is con- 

 cealed by the massive SPINAL MUSCLES. 



8. The Ribs. Count the RIBS on both sides, remembering that the 

 last is short. The normal number is thirteen. Call the attention of the 

 instructor to any anomaly in the number on either side. 



9. The Costal Cartilages. Select the sixth rib, the middle of the 

 normal series. Follow it to the sternum. At any spot between the 

 sternum and the middle of the dorso-ventral diameter of the thorax the 

 tracer-point is not resisted as in the part nearer the spine. This softer 

 ventral part is the COSTAL CARTILAGE. Its place of junction with the rib 

 may be determined either by testing with the point till the hard bone is 

 reached, or by scraping off the adherent muscle remnants at about the 

 place indicated in the figure, and noting the slight difference in color. 



\ 10. Sternal and Asternal Ribs. By manipulation determine that the first nine (rarely 

 eight) cartilages reach the sternum ; the corresponding ribs are hence called TRUE or 

 sternal, and the other four FALSE or asternal; the loth, nth and i2th cartilages are 

 attached, each to the one next cephalad, but the i3th lies free among the muscles and its 

 rib is called FLOATING. Compare with the arrangement in man. 



\ 1 1. Intercostal Muscles. Between the adjacent ribs and cartilages are INTERCOSTAL 

 MUSCLES ; their ectal layer has the'same general direction as the ectal muscle of the abdo- 

 men ; indeed they may be regarded as parts of the same muscle, the thoracic portion 

 interrupted at intervals by the ribs and cartilages. 



\ 12. The Sternum (breastbone). The cat's sternum is relatively longer, narrower and 

 more flexible than the human ; it consists of eight or nine segments, the short one, bear- 

 ing the letter M in Fig. i, being sometimes wanting. 



S 13. Opening the Right Thorax. This should be done as directed 

 along the lines indicated in Fig. i, and without pulling upon the parts, 

 lest certain membranes be cut or torn. 



a. With the scalpel divide the 6th cartilage near the rib, at about 

 the point of crossing of the heavy interrupted line on Fig. i as represent- 

 ed by the arrow ; the complete division is indicated by the separability 

 of the two parts. 



b. Hold the scalpel with its edge upward, and nearly horizontal so 

 as to form a slight angle with the thorax. Cautiously introduce the 

 point, not more than 5 mm., and divide the intercostal muscle just ceph- 

 alad. Then cut the next cartilage, and so continue till the second carti- 

 lage and first intercostal are divided. 



c. From the starting point cut catidad including the eleventh carti- 

 lage ; then trim the first intercostal along the margin of the first cartilage, 

 and with the scissors cut thence caudad, about i cm. from the sternum, 

 including the eighth cartilage. ' 



d. Before continuing the incision lift the portion of parietes thus 

 circumscribed, and note that the caudal end follows the curve of an ental 

 attachment (to the diaphragm) ; with the scissors cut along that curve, at 

 about r cm. from the attachment, to the point between the eleventh and 

 twelfth cartilages where the second incision stopped. The direction is 

 indicated approximately by the broken line in Fig. i crossing the gth, 

 mth and irth cartilages. Remove the piece, cartilages and intercostals, 

 so freed. 



