326 



DESIGN IN NATURE 



PLATE LXXXV (continued) 



Fig. Y. — Spiral shell with right-handed spiral whorl at the apex. The whorl consists of two parts which twist and plait into 

 each other. It resembles a right-handed spiral nebula (Plate viii., Pig. 4); spiral seeds (Fig. 11, D, B) ; spiral spermatozoa 

 (Plate xii., Fig. 1, E), &c. (the Author, 1859, and subsequently). 



Fig. 8. — Photograph of a dissection by the Author of the left ventricle of a sheep's heart showing the posterior surface and 

 apex of the left ventricle. The left ventricle of the heart of the mammal is composed of two sets of spiral muscular fibres. /, h, 

 the two sets of superficial spiral muscular fibres form the first external layer. They are seen involuting or turning into 

 the left apex to make a most exquisite whorl. Compare with Fig. 7 of this Plate. It is these two sets of fibres which are chiefly 

 concerned in the production of the two musculi papillares. The whorl formed by the two sets of spiral muscular fibres of the left 

 ventricle of the heart of the sheep is quite as distinct in the left ventricle of the human heart. 



B 



« .^ 



I'iG. 67. 



Fia. 67 (A, B, 0, D, E).— Represents a series of original dissections and drawings by the Author, showing the distribution of the 

 muscular fibres in the oesophagus, stomach, pyloric valve, and duodenum. 



A. Minute dissection of the muscular fibres of the oesophagus of the horse. In this dissection no fewer than seven layers of 

 muscular fibres can be made out. The distribution is similar to that met with in the left ventricle of the mammalian heart The 

 fibres are spirally arranged, and consist of three external sets, three internal sets, and a central layer. Proceeding from without 

 inwards the following are encountered— an external longitudinal layer (a, n', a'), an internal ditto (e), a slightly oblique external and 

 a slightly oblique internal layer which cross each other (h, V), a very oblique external and a very oblique internal layer which also 

 cross (c, c'), and a transverse or central layer (d, d'). This muscular mechanism is endowed with centrifugal and centripetal powers 

 whereby it opens before and closes behind the bolus in swallowing, with the result that the food is enforced into the stomach. The 

 scheme of the mechanism is indicated at D, E, of Fig. .3, Plate Ixxxiv. 



In vomiting, the swallowing process is reversed ; the arrangement working efi'ectively in both cases. One can eat swallow 

 and drink with the body inverted. ' ' 



The swallowing process is similar to that induced in the teat of the cow in milking. According to prevailing views the interior 

 of the oesophagus is irritable, and the bolus of food acts as a stimulus which causes the oesophagus to open and close rdternatelii and 

 so transmit it to the stomach. This cannot be the true explanation. The passage of the bolus into the stomach is a continuous 

 movement, and cannot possibly produce the alternating, interrupted movements which characterise the ojsophagus If moreover 

 the bolus acted as a stimulant it would cause contraction of the oesophagus in front instead of behind itself. The oesophagus would 



